Exploiting broad- and narrow-band images of the Hubble Space Telescope from near-UV to I-band
restframe, we study the star-forming clumps of six galaxies of the GASP sample undergoing strong
ram-pressure stripping (RPS). Clumps are detected in Hα and near-UV, tracing star formation on
different timescales. We consider clumps located in galaxy disks, in the stripped tails and those
formed in stripped gas but still close to the disk, called extraplanar. We detect 2406 Hα-selected
clumps (1708 in disks, 375 in extraplanar regions, and 323 in tails) and 3750 UV-selected clumps (2026
disk clumps, 825 extraplanar clumps and 899 tail clumps). Only ∼ 15% of star-forming clumps are
spatially resolved, meaning that most are smaller than ∼ 140 pc. We study the luminosity and size
distribution functions (LDFs and SDFs, respectively) and the luminosity-size relation. The average
LDF slope is 1.79 ± 0.09, while the average SDF slope is 3.1 ± 0.5. Results suggest the star formation
to be turbulence driven and scale-free, as in main-sequence galaxies. All the clumps, whether they are
in the disks or in the tails, have an enhanced Hα luminosity at a given size, compared to the clumps in
main-sequence galaxies. Indeed, their Hα luminosity is closer to that of clumps in starburst galaxies,
indicating that ram pressure is able to enhance the luminosity. No striking differences are found among
disk and tail clumps, suggesting that the different environments in which they are embedded play a
minor role in influencing the star formation.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
Detection of anisotropic satellite quenching in galaxy clusters up to z ∼ 1Sérgio Sacani
Satellite galaxies in the cluster environment are more likely to be quenched than galaxies in the general field. Recently, it has
been reported that satellite galaxy quenching depends on the orientation relative to their central galaxies: satellites along the
major axis of centrals are more likely to be quenched than those along the minor axis. In this paper, we report a detection
of such anisotropic quenching up to z ∼ 1 based on a large optically selected cluster catalogue constructed from the Hyper
Suprime-Cam Subaru Strategic Program. We calculate the quiescent satellite galaxy fraction as a function of orientation angle
measured from the major axis of central galaxies and find that the quiescent fractions at 0.25 < z < 1 are reasonably fitted
by sinusoidal functions with amplitudes of a few per cent. Anisotropy is clearer in inner regions (<r200m) of clusters and not
significant in cluster outskirts (>r200m). We also confirm that the observed anisotropy cannot be explained by differences in
local galaxy density or stellar mass distribution along the two axes. Quiescent fraction excesses between the two axes suggest
that the quenching efficiency contributing to the anisotropy is almost independent of stellar mass, at least down to our stellar
mass limit of M∗ = 1 × 1010 M. Finally, we argue that the physical origins of the observed anisotropy should have shorter
quenching time-scales than ∼ 1 Gyr, like ram-pressure stripping, because, for anisotropic quenching to be observed, satellites
must be quenched before their initial orientation angles are significantly changed.
The SAMI Galaxy Sur v ey: galaxy spin is more strongly correlated with stella...Sérgio Sacani
We use the SAMI Galaxy Surv e y to examine the drivers of galaxy spin, λR e , in a multidimensional parameter space including stellar mass, stellar population age (or specific star formation rate), and various environmental metrics (local density, halo mass, satellite versus central). Using a partial correlation analysis, we consistently find that age or specific star formation rate is the primary parameter correlating with spin. Light-weighted age and specific star formation rate are more strongly correlated with spin than mass-weighted age. In fact, across our sample, once the relation between light-weighted age and spin is accounted for, there is no significant residual correlation between spin and mass, or spin and environment. This result is strongly suggestive that the present-day environment only indirectly influences spin, via the removal of gas and star formation quenching. That is, environment affects age, then age affects spin. Older galaxies then have lower spin, either due to stars being born dynamically hotter at high redshift, or due to secular heating. Our results appear to rule out environmentally dependent dynamical heating (e.g. g alaxy–g alaxy interactions) being important, at least within 1 R e where our kinematic measurements are made. The picture is more complex when we only consider high-mass galaxies ( M ∗ ≳ 10 11 M ). While the age-spin relation is still strong for these high-mass galaxies, there is a residual environmental trend with central galaxies preferentially having lower spin, compared to satellites of the same age and mass. We argue that this trend is likely due to central galaxies being a preferred location for mergers.
Kinematics and simulations_of_the_stellar_stream_in_the_halo_of_the_umbrella_...Sérgio Sacani
This document summarizes a study of the stellar stream and substructures around the Umbrella Galaxy (NGC 4651). Deep imaging and spectroscopy were used to characterize the properties and kinematics of the stream. Tracer objects like globular clusters and planetary nebulae were identified and found to delineate a kinematically cold feature in position-velocity space. Dynamical modeling suggests the stream originated from the tidal disruption of a dwarf galaxy on a highly eccentric orbit about 6-10 billion years ago. This work demonstrates the feasibility of using discrete tracers to recover the kinematics and model the dynamics of low surface brightness stellar streams around distant galaxies.
The SPHERE view of three interacting twin disc systems in polarised lightSérgio Sacani
Dense stellar environments as hosts of ongoing star formation increase the probability of gravitational encounters among stellar
systems during the early stages of evolution. Stellar interaction may occur through non-recurring, hyperbolic or parabolic passages
(a so-called ‘fly-by’), through secular binary evolution, or through binary capture. In all three scenarios, the strong gravitational
perturbation is expected to manifest itself in the disc structures around the individual stars. Here, we present near-infrared
polarised light observations that were taken with the SPHERE/IRDIS instrument of three known interacting twin-disc systems:
AS 205, EM* SR 24, and FU Orionis. The scattered light exposes spirals likely caused by the gravitational interaction. On
a larger scale, we observe connecting filaments between the stars. We analyse their very complex polarised intensity and put
particular attention to the presence of multiple light sources in these systems. The local angle of linear polarisation indicates
the source whose light dominates the scattering process from the bridging region between the two stars. Further, we show
that the polarised intensity from scattering with multiple relevant light sources results from an incoherent summation of the
individuals’ contribution. This can produce nulls of polarised intensity in an image, as potentially observed in AS 205.We discuss
the geometry and content of the systems by comparing the polarised light observations with other data at similar resolution,
namely with ALMA continuum and gas emission. Collective observational data can constrain the systems’ geometry and stellar
trajectories, with the important potential to differentiate between dynamical scenarios of stellar interaction.
Exploring the nature and synchronicity of early cluster formation in the Larg...Sérgio Sacani
We analyse Hubble Space Telescope observations of six globular clusters in the Large Magel- lanic Cloud (LMC) from programme GO-14164 in Cycle 23. These are the deepest available observations of the LMC globular cluster population; their uniformity facilitates a precise comparison with globular clusters in the Milky Way. Measuring the magnitude of the main- sequence turn-off point relative to template Galactic globular clusters allows the relative ages of the clusters to be determined with a mean precision of 8.4 per cent, and down to 6 per cent for individual objects. We find that the mean age of our LMC cluster ensemble is identical to the mean age of the oldest metal-poor clusters in the Milky Way halo to 0.2 ± 0.4 Gyr. This provides the most sensitive test to date of the synchronicity of the earliest epoch of globular cluster formation in two independent galaxies. Horizontal branch magnitudes and subdwarf fitting to the main sequence allow us to determine distance estimates for each cluster and examine their geometric distribution in the LMC. Using two different methods, we find an average distance to the LMC of 18.52 ± 0.05.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
Detection of anisotropic satellite quenching in galaxy clusters up to z ∼ 1Sérgio Sacani
Satellite galaxies in the cluster environment are more likely to be quenched than galaxies in the general field. Recently, it has
been reported that satellite galaxy quenching depends on the orientation relative to their central galaxies: satellites along the
major axis of centrals are more likely to be quenched than those along the minor axis. In this paper, we report a detection
of such anisotropic quenching up to z ∼ 1 based on a large optically selected cluster catalogue constructed from the Hyper
Suprime-Cam Subaru Strategic Program. We calculate the quiescent satellite galaxy fraction as a function of orientation angle
measured from the major axis of central galaxies and find that the quiescent fractions at 0.25 < z < 1 are reasonably fitted
by sinusoidal functions with amplitudes of a few per cent. Anisotropy is clearer in inner regions (<r200m) of clusters and not
significant in cluster outskirts (>r200m). We also confirm that the observed anisotropy cannot be explained by differences in
local galaxy density or stellar mass distribution along the two axes. Quiescent fraction excesses between the two axes suggest
that the quenching efficiency contributing to the anisotropy is almost independent of stellar mass, at least down to our stellar
mass limit of M∗ = 1 × 1010 M. Finally, we argue that the physical origins of the observed anisotropy should have shorter
quenching time-scales than ∼ 1 Gyr, like ram-pressure stripping, because, for anisotropic quenching to be observed, satellites
must be quenched before their initial orientation angles are significantly changed.
The SAMI Galaxy Sur v ey: galaxy spin is more strongly correlated with stella...Sérgio Sacani
We use the SAMI Galaxy Surv e y to examine the drivers of galaxy spin, λR e , in a multidimensional parameter space including stellar mass, stellar population age (or specific star formation rate), and various environmental metrics (local density, halo mass, satellite versus central). Using a partial correlation analysis, we consistently find that age or specific star formation rate is the primary parameter correlating with spin. Light-weighted age and specific star formation rate are more strongly correlated with spin than mass-weighted age. In fact, across our sample, once the relation between light-weighted age and spin is accounted for, there is no significant residual correlation between spin and mass, or spin and environment. This result is strongly suggestive that the present-day environment only indirectly influences spin, via the removal of gas and star formation quenching. That is, environment affects age, then age affects spin. Older galaxies then have lower spin, either due to stars being born dynamically hotter at high redshift, or due to secular heating. Our results appear to rule out environmentally dependent dynamical heating (e.g. g alaxy–g alaxy interactions) being important, at least within 1 R e where our kinematic measurements are made. The picture is more complex when we only consider high-mass galaxies ( M ∗ ≳ 10 11 M ). While the age-spin relation is still strong for these high-mass galaxies, there is a residual environmental trend with central galaxies preferentially having lower spin, compared to satellites of the same age and mass. We argue that this trend is likely due to central galaxies being a preferred location for mergers.
Kinematics and simulations_of_the_stellar_stream_in_the_halo_of_the_umbrella_...Sérgio Sacani
This document summarizes a study of the stellar stream and substructures around the Umbrella Galaxy (NGC 4651). Deep imaging and spectroscopy were used to characterize the properties and kinematics of the stream. Tracer objects like globular clusters and planetary nebulae were identified and found to delineate a kinematically cold feature in position-velocity space. Dynamical modeling suggests the stream originated from the tidal disruption of a dwarf galaxy on a highly eccentric orbit about 6-10 billion years ago. This work demonstrates the feasibility of using discrete tracers to recover the kinematics and model the dynamics of low surface brightness stellar streams around distant galaxies.
The SPHERE view of three interacting twin disc systems in polarised lightSérgio Sacani
Dense stellar environments as hosts of ongoing star formation increase the probability of gravitational encounters among stellar
systems during the early stages of evolution. Stellar interaction may occur through non-recurring, hyperbolic or parabolic passages
(a so-called ‘fly-by’), through secular binary evolution, or through binary capture. In all three scenarios, the strong gravitational
perturbation is expected to manifest itself in the disc structures around the individual stars. Here, we present near-infrared
polarised light observations that were taken with the SPHERE/IRDIS instrument of three known interacting twin-disc systems:
AS 205, EM* SR 24, and FU Orionis. The scattered light exposes spirals likely caused by the gravitational interaction. On
a larger scale, we observe connecting filaments between the stars. We analyse their very complex polarised intensity and put
particular attention to the presence of multiple light sources in these systems. The local angle of linear polarisation indicates
the source whose light dominates the scattering process from the bridging region between the two stars. Further, we show
that the polarised intensity from scattering with multiple relevant light sources results from an incoherent summation of the
individuals’ contribution. This can produce nulls of polarised intensity in an image, as potentially observed in AS 205.We discuss
the geometry and content of the systems by comparing the polarised light observations with other data at similar resolution,
namely with ALMA continuum and gas emission. Collective observational data can constrain the systems’ geometry and stellar
trajectories, with the important potential to differentiate between dynamical scenarios of stellar interaction.
Exploring the nature and synchronicity of early cluster formation in the Larg...Sérgio Sacani
We analyse Hubble Space Telescope observations of six globular clusters in the Large Magel- lanic Cloud (LMC) from programme GO-14164 in Cycle 23. These are the deepest available observations of the LMC globular cluster population; their uniformity facilitates a precise comparison with globular clusters in the Milky Way. Measuring the magnitude of the main- sequence turn-off point relative to template Galactic globular clusters allows the relative ages of the clusters to be determined with a mean precision of 8.4 per cent, and down to 6 per cent for individual objects. We find that the mean age of our LMC cluster ensemble is identical to the mean age of the oldest metal-poor clusters in the Milky Way halo to 0.2 ± 0.4 Gyr. This provides the most sensitive test to date of the synchronicity of the earliest epoch of globular cluster formation in two independent galaxies. Horizontal branch magnitudes and subdwarf fitting to the main sequence allow us to determine distance estimates for each cluster and examine their geometric distribution in the LMC. Using two different methods, we find an average distance to the LMC of 18.52 ± 0.05.
Star formation at the smallest scales; A JWST study of the clump populations ...Sérgio Sacani
We present the clump populations detected in 18 lensed galaxies at redshifts 1 to 8.5 within the lensing cluster field SMACS0723.
The recent JWST Early Release Observations of this poorly known region of the sky have revealed numerous point-like sources
within and surrounding their host galaxies, undetected in the shallower HST images. We use JWST multiband photometry and
the lensing model of this galaxy cluster to estimate the intrinsic sizes and magnitudes of the stellar clumps. We derive optical
restframe effective radii from <10 to hundreds pc and masses ranging from ∼ 105
to 109 M, overlapping with massive star
clusters in the local universe. Clump ages range from 1 Myr to 1 Gyr. We compare the crossing time to the age of the clumps
and determine that between 45 and 60 % of the detected clumps are consistent with being gravitationally bound. On average,
the dearth of Gyr old clumps suggests that the dissolution time scales are shorter than 1 Gyr. We see a significant increase in the
luminosity (mass) surface density of the clumps with redshift. Clumps in reionisation era galaxies have stellar densities higher
than star clusters in the local universe. We zoom in into single galaxies at redshift < 6 and find for two galaxies, the Sparkler and
the Firework, that their star clusters/clumps show distinctive colour distributions and location surrounding their host galaxy that
are compatible with being accredited or formed during merger events. The ages of some of the compact clusters are between
1 and 4 Gyr, e.g., globular cluster precursors formed around 9-12 Gyr ago. Our study, conducted on a small sample of galaxies,
shows the potential of JWST observations for understanding the conditions under which star clusters form in rapidly evolving
galaxies.
Asymmetrical tidal tails of open star clusters: stars crossing their cluster’...Sérgio Sacani
The document discusses asymmetrical tidal tails observed around five open star clusters, which challenges Newtonian gravity. It summarizes how tidal tails form as stars escape clusters due to energy equipartition. Observations of the Hyades, Praesepe, Coma Berenices, COIN-Gaia 13, and NGC 752 clusters found more stars in the leading tidal tails within 50 pc of the clusters. Simulations show that in Newtonian gravity, tidal tails should be symmetrical, but asymmetries can arise in Milgromian dynamics. Future work is needed to better map tidal tails and develop Milgromian simulations.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
The document summarizes a study of an extreme molecular cloud in the Antennae galaxies that has properties consistent with forming a globular cluster. ALMA observations reveal a cloud with a radius of 24 pc and mass greater than 5 million solar masses. While capable of forming a globular cluster, a lack of associated thermal radio emission indicates star formation has not yet begun to alter the environment, suggesting the cloud is in an early stage of evolution. For the cloud to be confined as observed, an external pressure over 10,000 times greater than typical interstellar pressure is required, supporting the theory that high pressures are needed to form globular clusters in extreme environments like mergers.
GASP XVII. HI imaging of the jellyfish galaxy JO206: gas stripping and enhanc...Sérgio Sacani
We present VLA H I observations of JO206, a prototypical ram-pressure stripped
galaxy in the GASP sample. This massive galaxy (M∗ = 8.5 × 1010 M) is located
at a redshift of z = 0.0513, near the centre of the low-mass galaxy cluster, IIZw108
(σ ∼ 575 km s−1
). JO206 is characterised by a long tail (≥90 kpc) of ionised gas
stripped away by ram-pressure. We find a similarly long H I tail in the same direction
as the ionised gas tail and measure a total H I mass of 3.2×109 M. This is about half
the expected H I mass given the stellar mass and surface density of JO206. A total of
1.8 × 109 M (60%) of the detected H I is in the gas stripped tail. An analysis of the
star formation rate shows that the galaxy is forming more stars compared to galaxies
with the same stellar and H I mass. On average we find a H I gas depletion time of
∼0.5 Gyr which is about four times shorter than that of “normal” spiral galaxies.
We performed a spatially resolved analysis of the relation between star formation rate
density and gas density in the disc and tail of the galaxy at the resolution of our H I
data. The star formation efficiency of the disc is about 10 times higher than that of
the tail at fixed H I surface densities. Both the inner and outer parts of JO206 show
an enhanced star formation compared to regions of similar H I surface density in field
galaxies. The enhanced star formation is due to ram-pressure stripping during the
galaxy’s first infall into the cluster.
A High-mass, Young Star-forming Core Escaping from Its Parental FilamentSérgio Sacani
We studied the unique kinematic properties in massive filament G352.63-1.07 at 103 au spatial scale with the dense
molecular tracers observed with the Atacama Large Millimeter/submillimeter Array. We find the central massive
core M1 (12 Me) being separated from the surrounding filament with a velocity difference of
- =- - v vsys 2 km s 1 and a transverse separation within 3″. Meanwhile, as shown in multiple dense-gas
tracers, M1 has a spatial extension closely aligned with the main filament and is connected to the filament toward
both its ends. M1 thus represents a very beginning state for a massive, young star-forming core escaping from the
parental filament, within a timescale of ∼4000 yr. Based on its kinetic energy (3.5 × 1044 erg), the core escape is
unlikely solely due to the original filament motion or magnetic field but requires more energetic events such as a
rapid intense anisotropic collapse. The released energy also seems to noticeably increase the environmental
turbulence. This may help the filament to become stabilized again.
This pilot survey used modest aperture telescopes to image 8 nearby spiral galaxies in order to search for stellar tidal streams. Ultra-deep imaging revealed 6 previously undetected extensive (up to 30 kpc) stellar structures likely from tidally disrupted satellites. A diversity of tidal feature morphologies was found, including great circle-like streams, remote shells, and jets emerging from disks. Simulations predict tidal debris should be common and match the observed variety, providing evidence minor mergers have shaped disk galaxies since z=1.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
MUSE sneaks a peek at extreme ram-pressure stripping events. I. A kinematic s...Sérgio Sacani
- MUSE observations of the galaxy ESO137-001 reveal an extended gaseous tail over 30 kpc long traced by H-alpha emission, providing evidence of an extreme ram pressure stripping event as the galaxy falls into the massive Norma galaxy cluster.
- Analysis of the H-alpha kinematics and stellar velocity field show that ram pressure has removed the interstellar medium from the outer disk while the primary tail is still fed by gas from the galaxy center, with gravitational interactions not appearing to be the main mechanism of gas removal.
- The stripped gas retains evidence of the disk's rotational velocity out to around 20 kpc downstream, indicating the galaxy is moving radially along the plane of the sky, while
This document describes observations of the galaxy ESO137-001 using the MUSE instrument on the VLT. The key points are:
1) MUSE observations reveal an extended gas tail stretching over 30 kpc from the galaxy, tracing ongoing ram pressure stripping as it falls into the Norma galaxy cluster.
2) Analysis of the gas kinematics and stellar velocity field show that ram pressure has removed the interstellar medium from the outer disk while the primary tail is still fed by gas from the galaxy center.
3) The stripped gas retains evidence of the disk's rotational velocity out to 20 kpc downstream, indicating the galaxy is moving radially through the cluster. Beyond this the gas shows greater turbulence,
Detecting a disc bending wave in a barred-spiral galaxy at redshift 4.4Sérgio Sacani
The recent discovery of barred spiral galaxies in the early Universe (z > 2) poses questions of how these structures form and
how they influence galaxy evolution in the early Universe. In this study, we investigate the morphology and kinematics of the
far-infrared (FIR) continuum and [C II] emission in BRI1335-0417 at z ≈ 4.4 from ALMA observations. The variations in
position angle and ellipticity of the isophotes show the characteristic signature of a barred galaxy. The bar, 3.3+0.2 −0.2 kpc long in
radius and bridging the previously identified two-armed spiral, is evident in both [C II] and FIR images, driving the galaxy’s rapid
evolution by channelling gas towards the nucleus. Fourier analysis of the [C II] velocity field reveals an unambiguous kinematic
m = 2 mode with a line-of-sight velocity amplitude of up to ∼30–40 km s−1; a plausible explanation is the disc’s vertical bending
mode triggered by external perturbation, which presumably induced the high star formation rate and the bar/spiral structure. The
bar identified in [C II] and FIR images of the gas-rich disc galaxy ( 70 per cent of the total mass within radius R ≈ 2.2 disc
scale lengths) suggests a new perspective of early bar formation in high redshift gas-rich galaxies – a gravitationally unstable
gas-rich disc creating a star-forming gaseous bar, rather than a stellar bar emerging from a pre-existing stellar disc. This may
explain the prevalent bar-like structures seen in FIR images of high-redshift submillimeter galaxies.
DYNAMICAL ANALYSIS OF THE DARK MATTER AND CENTRAL BLACK HOLE MASS IN THE DWAR...Sérgio Sacani
We measure the central kinematics for the dwarf spheroidal galaxy Leo I using integrated-light measurements and
previously published data. We find a steady rise in the velocity dispersion from 30000 into the center. The integratedlight kinematics provide a velocity dispersion of 11.76±0.66 km s−1
inside 7500. After applying appropriate corrections
to crowding in the central regions, we achieve consistent velocity dispersion values using velocities from individual stars.
Crowding corrections need to be applied when targeting individual stars in high density stellar environments. From
integrated light, we measure the surface brightness profile and find a shallow cusp towards the center. Axisymmetric,
orbit-based models measure the stellar mass-to-light ratio, black hole mass and parameters for a dark matter halo. At
large radii it is important to consider possible tidal effects from the Milky Way so we include a variety of assumptions
regarding the tidal radius. For every set of assumptions, models require a central black hole consistent with a mass
3.3 ± 2×106 M. The no-black-hole case for any of our assumptions is excluded at over 95% significance, with
6.4 < ∆χ
2 < 14. A black hole of this mass would have significant effect on dwarf galaxy formation and evolution.
The dark halo parameters are heavily affected by the assumptions for the tidal radii, with the circular velocity only
constrained to be above 30 km s−1
. Reasonable assumptions for the tidal radius result in stellar orbits consistent with
an isotropic distribution in the velocities. These more realistic models only show strong constraints for the mass of
the central black hole.
The galaxy morphology–density relation in the EAGLE simulationSérgio Sacani
This study uses a convolutional neural network trained on observed galaxy images to classify galaxies in the EAGLE cosmological hydrodynamical simulation by visual morphology. It finds that EAGLE reproduces both the observed relationships between galaxy morphology and local density (the morphology-density relation), and between morphology and galaxy mass. Through analysis of the simulations, the key processes identified as driving the morphology-density relation are: (1) the transformation of disc galaxies into lenticular galaxies via gas stripping in dense environments, (2) the formation of lenticular galaxies through merger-induced black hole feedback in less dense environments, and (3) the increasing fraction of more elliptical, high-mass galaxies at higher densities.
Os astrônomos descobriram um processo único sobre como as maiores galáxias elípticas do universo continuam gerando estrelas muito tempo depois do anos de pico de nascimentos estelares. A alta resolução e a sensibilidade à radiação ultravioleta do Hubble, permitiu aos astrônomos observarem nós brilhantes de estrelas azuis, quentes, se formando juntamente com jatos de buracos negros ativos encontrados nos centros das gigantescas galáxias elípticas.
Combinando dados do Huubble com observações feitas por um conjunto de telescópios baseados tanto em Terra como no espaço, duas equipes independentes descobriram que os jatos dos buracos negros, e as estrelas recém-nascidas são todos partes de um ciclo auto-regulado. Jatos de alta energia atirados do buraco negro aquecem um halo de gás circulante, controlando a taxa com a qual o gás esfria e cai na galáxia.
“Pense no gás ao redor da galáxia como uma atmosfera”, explicou o líder do primeiro estudo, Megan Donahue, da Universidade Estadual do Michigan. “Essa atmosfera pode conter material em diferentes estados, do mesmo modo que a nossa atmosfera tem gás, nuvens e chuva. O que nós estamos vendo é um processo parecido com uma tempestade. À medida que os jatos impulsionam o gás para fora do centro da galáxia, parte do gás esfria e precipita em aglomerados frios que caem de volta para o centro da galáxia como gotas de chuvas”.
“As gotas de chuva eventualmente esfriam o suficiente para tornar-se nuvens de formação de estrelas de gás frio molecular, e a capacidade de observar no ultravioleta distante do Hubble, nos permitiu observar diretamente esses chuviscos de formação de estrelas”, explicou o líder do segundo estudo, Grant Tremblay, da Universidade de Yale. “Nós sabemos que esses chuviscos estão linkados com os jatos, pois eles foram encontrados em filamentos que se dobram ao redor dos jatos, ou abraçam as bordas de bolhas gigantes que os jatos inflaram”, disse Tremblay. “E eles terminam fazendo um redemoinho de gás de formação de estrelas ao redor do buraco negro central”.
Stellar-mass black holes in the Hyades star cluster?Sérgio Sacani
Astrophysical models of binary-black hole mergers in the Universe require a significant fraction of stellar-mass black holes (BHs)
to receive negligible natal kicks to explain the gravitational wave detections. This implies that BHs should be retained even in
open clusters with low escape velocities (≲ 1 km/s). We search for signatures of the presence of BHs in the nearest open cluster
to the Sun – the Hyades – by comparing density profiles of direct 𝑁-body models to data from Gaia. The observations are best
reproduced by models with 2−3 BHs at present. Models that never possessed BHs have an half-mass radius ∼ 30% smaller than
the observed value, while those where the last BHs were ejected recently (≲ 150 Myr ago) can still reproduce the density profile.
In 50% of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at ∼ 103 yr, making
them unlikely to be found through velocity variations. We look for potential BH companions through large Gaia astrometric and
spectroscopic errors, identifying 56 binary candidates - none of which consistent with a massive compact companion. Models
with 2 − 3 BHs have an elevated central velocity dispersion, but observations can not yet discriminate. We conclude that the
present-day structure of the Hyades requires a significant fraction of BHs to receive natal kicks smaller than the escape velocity
of ∼ 3 km s−1
at the time of BH formation and that the nearest BHs to the Sun are in, or near, Hyades.
Disks of Stars in the Galactic Center Triggered by Tidal Disruption EventsSérgio Sacani
This document proposes that tidal disruption events (TDEs) from wandering stars could trigger episodes of positive star formation feedback in the Galactic Center, providing an explanation for the observed disks of young stars near Sgr A*. When a star is tidally disrupted by the supermassive black hole, the resulting jet compresses gas clouds to densities high enough to resist tidal forces and form stars within the disk plane perpendicular to the jet. The estimated rate of jetted TDEs is consistent with the age of the disk stars. This mechanism predicts a random orientation for each disk and the potential for multiple misaligned disks from separate TDE events.
Galaxy interactions are the dominant trigger for local type 2 quasarsSérgio Sacani
The triggering mechanism for the most luminous, quasar-like active galactic nuclei (AGN) remains a source of debate, with
some studies favouring triggering via galaxy mergers, but others finding little evidence to support this mechanism. Here, we
present deep Isaac Newton Telescope/Wide Field Camera imaging observations of a complete sample of 48 optically selected
type 2 quasars – the QSOFEED sample (L[O III] > 108.5 L; z < 0.14). Based on visual inspection by eight classifiers, we find
clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with
65+6
−7 per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared
with only 22+5
−4 per cent of a stellar-mass- and redshift-matched comparison sample of non-AGN galaxies – a 5σ difference. The
type 2 quasar hosts are a factor of 3.0+0.5 −0.8 more likely to be morphologically disturbed than their matched non-AGN counterparts,
similar to our previous results for powerful 3CR radio AGN of comparable [O III] emission-line luminosity and redshift. In
contrast to the idea that quasars are triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only become
visible post-coalescence, the majority of morphologically disturbed type 2 quasar sources in our sample are observed in the
pre-coalescence phase (61+8
−9 per cent). We argue that much of the apparent ambiguity that surrounds observational results in this
field is a result of differences in the surface brightness depths of the observations, combined with the effects of cosmological
surface brightness dimming.
We present deep optical images of the Large and Small Magellanic Clouds (LMC and SMC) using
a low cost telephoto lens with a wide field of view to explore stellar substructure in the outskirts
of the stellar disk of the LMC (r < 10 degrees from the center). These data have higher resolution
than existing star count maps, and highlight the existence of stellar arcs and multiple spiral arms in
the northern periphery, with no comparable counterparts in the South. We compare these data to
detailed simulations of the LMC disk outskirts, following interactions with its low mass companion,
the SMC. We consider interaction in isolation and with the inclusion of the Milky Way tidal field.
The simulations are used to assess the origin of the northern structures, including also the low density
stellar arc recently identified in the DES data by Mackey et al. (2015) at ∼ 15 degrees. We conclude
that repeated close interactions with the SMC are primarily responsible for the asymmetric stellar
structures seen in the periphery of the LMC. The orientation and density of these arcs can be used to
constrain the LMC’s interaction history with and impact parameter of the SMC. More generally, we
find that such asymmetric structures should be ubiquitous about pairs of dwarfs and can persist for
1-2 Gyr even after the secondary merges entirely with the primary. As such, the lack of a companion
around a Magellanic Irregular does not disprove the hypothesis that their asymmetric structures are
driven by dwarf-dwarf interactions.
Formation of low mass protostars and their circumstellar disksSérgio Sacani
Understanding circumstellar disks is of prime importance in astrophysics, however, their birth process remains poorly constrained due to observational and numerical challenges. Recent numerical works have shown that the small-scale physics, often wrapped into a sub-grid model, play a crucial role in disk formation and evolution. This calls for a combined approach in which both the protostar and circumstellar disk are studied in concert. Aims. We aim to elucidate the small scale physics and constrain sub-grid parameters commonly chosen in the literature by resolving the star-disk interaction. Methods. We carry out a set of very high resolution 3D radiative-hydrodynamics simulations that self-consistently describe the collapse of a turbulent dense molecular cloud core to stellar densities. We study the birth of the protostar, the circumstellar disk, and its early evolution (< 6 yr after protostellar formation). Results. Following the second gravitational collapse, the nascent protostar quickly reaches breakup velocity and sheds its surface material, thus forming a hot (∼ 103 K), dense, and highly flared circumstellar disk. The protostar is embedded within the disk, such that material can flow without crossing any shock fronts. The circumstellar disk mass quickly exceeds that of the protostar, and its kinematics are dominated by self-gravity. Accretion onto the disk is highly anisotropic, and accretion onto the protostar mainly occurs through material that slides on the disk surface. The polar mass flux is negligible in comparison. The radiative behavior also displays a strong anisotropy, as the polar accretion shock is shown to be supercritical whereas its equatorial counterpart is subcritical. We also f ind a remarkable convergence of our results with respect to initial conditions. Conclusions. These results reveal the structure and kinematics in the smallest spatial scales relevant to protostellar and circumstellar disk evolution. They can be used to describe accretion onto regions commonly described by sub-grid models in simulations studying larger scale physics.
DISCOVERY OF A GALAXY CLUSTER WITH A VIOLENTLY STARBURSTING CORE AT z = 2:506Sérgio Sacani
We report the discovery of a remarkable concentration of massive galaxies with extended X-ray
emission at zspec = 2:506, which contains 11 massive (M & 1011M) galaxies in the central 80kpc
region (11.6 overdensity). We have spectroscopically conrmed 17 member galaxies with 11 from CO
and the remaining ones from H. The X-ray luminosity, stellar mass content and velocity dispersion
all point to a collapsed, cluster-sized dark matter halo with mass M200c = 1013:90:2M, making it
the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this
structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive
galaxies classied as quiescent. The star formation rate (SFR) in the 80kpc core reaches 3400 M
yr 1 with a gas depletion time of 200 Myr, suggesting that we caught this cluster in rapid build-up
of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst
fraction ( 25%, compared to 3%-5% in the eld). The presence of both a collapsed, cluster-sized
halo and a predominant population of massive SFGs suggests that this structure could represent an
important transition phase between protoclusters and mature clusters. It provides evidence that the
main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster,
providing new insights into massive cluster formation at early epochs. The large integrated stellar
mass at such high redshift challenges our understanding of massive cluster formation.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
More Related Content
Similar to HST imaging of star-forming clumps in 6 GASP ram-pressure stripped galaxies
Star formation at the smallest scales; A JWST study of the clump populations ...Sérgio Sacani
We present the clump populations detected in 18 lensed galaxies at redshifts 1 to 8.5 within the lensing cluster field SMACS0723.
The recent JWST Early Release Observations of this poorly known region of the sky have revealed numerous point-like sources
within and surrounding their host galaxies, undetected in the shallower HST images. We use JWST multiband photometry and
the lensing model of this galaxy cluster to estimate the intrinsic sizes and magnitudes of the stellar clumps. We derive optical
restframe effective radii from <10 to hundreds pc and masses ranging from ∼ 105
to 109 M, overlapping with massive star
clusters in the local universe. Clump ages range from 1 Myr to 1 Gyr. We compare the crossing time to the age of the clumps
and determine that between 45 and 60 % of the detected clumps are consistent with being gravitationally bound. On average,
the dearth of Gyr old clumps suggests that the dissolution time scales are shorter than 1 Gyr. We see a significant increase in the
luminosity (mass) surface density of the clumps with redshift. Clumps in reionisation era galaxies have stellar densities higher
than star clusters in the local universe. We zoom in into single galaxies at redshift < 6 and find for two galaxies, the Sparkler and
the Firework, that their star clusters/clumps show distinctive colour distributions and location surrounding their host galaxy that
are compatible with being accredited or formed during merger events. The ages of some of the compact clusters are between
1 and 4 Gyr, e.g., globular cluster precursors formed around 9-12 Gyr ago. Our study, conducted on a small sample of galaxies,
shows the potential of JWST observations for understanding the conditions under which star clusters form in rapidly evolving
galaxies.
Asymmetrical tidal tails of open star clusters: stars crossing their cluster’...Sérgio Sacani
The document discusses asymmetrical tidal tails observed around five open star clusters, which challenges Newtonian gravity. It summarizes how tidal tails form as stars escape clusters due to energy equipartition. Observations of the Hyades, Praesepe, Coma Berenices, COIN-Gaia 13, and NGC 752 clusters found more stars in the leading tidal tails within 50 pc of the clusters. Simulations show that in Newtonian gravity, tidal tails should be symmetrical, but asymmetries can arise in Milgromian dynamics. Future work is needed to better map tidal tails and develop Milgromian simulations.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
The document summarizes a study of an extreme molecular cloud in the Antennae galaxies that has properties consistent with forming a globular cluster. ALMA observations reveal a cloud with a radius of 24 pc and mass greater than 5 million solar masses. While capable of forming a globular cluster, a lack of associated thermal radio emission indicates star formation has not yet begun to alter the environment, suggesting the cloud is in an early stage of evolution. For the cloud to be confined as observed, an external pressure over 10,000 times greater than typical interstellar pressure is required, supporting the theory that high pressures are needed to form globular clusters in extreme environments like mergers.
GASP XVII. HI imaging of the jellyfish galaxy JO206: gas stripping and enhanc...Sérgio Sacani
We present VLA H I observations of JO206, a prototypical ram-pressure stripped
galaxy in the GASP sample. This massive galaxy (M∗ = 8.5 × 1010 M) is located
at a redshift of z = 0.0513, near the centre of the low-mass galaxy cluster, IIZw108
(σ ∼ 575 km s−1
). JO206 is characterised by a long tail (≥90 kpc) of ionised gas
stripped away by ram-pressure. We find a similarly long H I tail in the same direction
as the ionised gas tail and measure a total H I mass of 3.2×109 M. This is about half
the expected H I mass given the stellar mass and surface density of JO206. A total of
1.8 × 109 M (60%) of the detected H I is in the gas stripped tail. An analysis of the
star formation rate shows that the galaxy is forming more stars compared to galaxies
with the same stellar and H I mass. On average we find a H I gas depletion time of
∼0.5 Gyr which is about four times shorter than that of “normal” spiral galaxies.
We performed a spatially resolved analysis of the relation between star formation rate
density and gas density in the disc and tail of the galaxy at the resolution of our H I
data. The star formation efficiency of the disc is about 10 times higher than that of
the tail at fixed H I surface densities. Both the inner and outer parts of JO206 show
an enhanced star formation compared to regions of similar H I surface density in field
galaxies. The enhanced star formation is due to ram-pressure stripping during the
galaxy’s first infall into the cluster.
A High-mass, Young Star-forming Core Escaping from Its Parental FilamentSérgio Sacani
We studied the unique kinematic properties in massive filament G352.63-1.07 at 103 au spatial scale with the dense
molecular tracers observed with the Atacama Large Millimeter/submillimeter Array. We find the central massive
core M1 (12 Me) being separated from the surrounding filament with a velocity difference of
- =- - v vsys 2 km s 1 and a transverse separation within 3″. Meanwhile, as shown in multiple dense-gas
tracers, M1 has a spatial extension closely aligned with the main filament and is connected to the filament toward
both its ends. M1 thus represents a very beginning state for a massive, young star-forming core escaping from the
parental filament, within a timescale of ∼4000 yr. Based on its kinetic energy (3.5 × 1044 erg), the core escape is
unlikely solely due to the original filament motion or magnetic field but requires more energetic events such as a
rapid intense anisotropic collapse. The released energy also seems to noticeably increase the environmental
turbulence. This may help the filament to become stabilized again.
This pilot survey used modest aperture telescopes to image 8 nearby spiral galaxies in order to search for stellar tidal streams. Ultra-deep imaging revealed 6 previously undetected extensive (up to 30 kpc) stellar structures likely from tidally disrupted satellites. A diversity of tidal feature morphologies was found, including great circle-like streams, remote shells, and jets emerging from disks. Simulations predict tidal debris should be common and match the observed variety, providing evidence minor mergers have shaped disk galaxies since z=1.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
MUSE sneaks a peek at extreme ram-pressure stripping events. I. A kinematic s...Sérgio Sacani
- MUSE observations of the galaxy ESO137-001 reveal an extended gaseous tail over 30 kpc long traced by H-alpha emission, providing evidence of an extreme ram pressure stripping event as the galaxy falls into the massive Norma galaxy cluster.
- Analysis of the H-alpha kinematics and stellar velocity field show that ram pressure has removed the interstellar medium from the outer disk while the primary tail is still fed by gas from the galaxy center, with gravitational interactions not appearing to be the main mechanism of gas removal.
- The stripped gas retains evidence of the disk's rotational velocity out to around 20 kpc downstream, indicating the galaxy is moving radially along the plane of the sky, while
This document describes observations of the galaxy ESO137-001 using the MUSE instrument on the VLT. The key points are:
1) MUSE observations reveal an extended gas tail stretching over 30 kpc from the galaxy, tracing ongoing ram pressure stripping as it falls into the Norma galaxy cluster.
2) Analysis of the gas kinematics and stellar velocity field show that ram pressure has removed the interstellar medium from the outer disk while the primary tail is still fed by gas from the galaxy center.
3) The stripped gas retains evidence of the disk's rotational velocity out to 20 kpc downstream, indicating the galaxy is moving radially through the cluster. Beyond this the gas shows greater turbulence,
Detecting a disc bending wave in a barred-spiral galaxy at redshift 4.4Sérgio Sacani
The recent discovery of barred spiral galaxies in the early Universe (z > 2) poses questions of how these structures form and
how they influence galaxy evolution in the early Universe. In this study, we investigate the morphology and kinematics of the
far-infrared (FIR) continuum and [C II] emission in BRI1335-0417 at z ≈ 4.4 from ALMA observations. The variations in
position angle and ellipticity of the isophotes show the characteristic signature of a barred galaxy. The bar, 3.3+0.2 −0.2 kpc long in
radius and bridging the previously identified two-armed spiral, is evident in both [C II] and FIR images, driving the galaxy’s rapid
evolution by channelling gas towards the nucleus. Fourier analysis of the [C II] velocity field reveals an unambiguous kinematic
m = 2 mode with a line-of-sight velocity amplitude of up to ∼30–40 km s−1; a plausible explanation is the disc’s vertical bending
mode triggered by external perturbation, which presumably induced the high star formation rate and the bar/spiral structure. The
bar identified in [C II] and FIR images of the gas-rich disc galaxy ( 70 per cent of the total mass within radius R ≈ 2.2 disc
scale lengths) suggests a new perspective of early bar formation in high redshift gas-rich galaxies – a gravitationally unstable
gas-rich disc creating a star-forming gaseous bar, rather than a stellar bar emerging from a pre-existing stellar disc. This may
explain the prevalent bar-like structures seen in FIR images of high-redshift submillimeter galaxies.
DYNAMICAL ANALYSIS OF THE DARK MATTER AND CENTRAL BLACK HOLE MASS IN THE DWAR...Sérgio Sacani
We measure the central kinematics for the dwarf spheroidal galaxy Leo I using integrated-light measurements and
previously published data. We find a steady rise in the velocity dispersion from 30000 into the center. The integratedlight kinematics provide a velocity dispersion of 11.76±0.66 km s−1
inside 7500. After applying appropriate corrections
to crowding in the central regions, we achieve consistent velocity dispersion values using velocities from individual stars.
Crowding corrections need to be applied when targeting individual stars in high density stellar environments. From
integrated light, we measure the surface brightness profile and find a shallow cusp towards the center. Axisymmetric,
orbit-based models measure the stellar mass-to-light ratio, black hole mass and parameters for a dark matter halo. At
large radii it is important to consider possible tidal effects from the Milky Way so we include a variety of assumptions
regarding the tidal radius. For every set of assumptions, models require a central black hole consistent with a mass
3.3 ± 2×106 M. The no-black-hole case for any of our assumptions is excluded at over 95% significance, with
6.4 < ∆χ
2 < 14. A black hole of this mass would have significant effect on dwarf galaxy formation and evolution.
The dark halo parameters are heavily affected by the assumptions for the tidal radii, with the circular velocity only
constrained to be above 30 km s−1
. Reasonable assumptions for the tidal radius result in stellar orbits consistent with
an isotropic distribution in the velocities. These more realistic models only show strong constraints for the mass of
the central black hole.
The galaxy morphology–density relation in the EAGLE simulationSérgio Sacani
This study uses a convolutional neural network trained on observed galaxy images to classify galaxies in the EAGLE cosmological hydrodynamical simulation by visual morphology. It finds that EAGLE reproduces both the observed relationships between galaxy morphology and local density (the morphology-density relation), and between morphology and galaxy mass. Through analysis of the simulations, the key processes identified as driving the morphology-density relation are: (1) the transformation of disc galaxies into lenticular galaxies via gas stripping in dense environments, (2) the formation of lenticular galaxies through merger-induced black hole feedback in less dense environments, and (3) the increasing fraction of more elliptical, high-mass galaxies at higher densities.
Os astrônomos descobriram um processo único sobre como as maiores galáxias elípticas do universo continuam gerando estrelas muito tempo depois do anos de pico de nascimentos estelares. A alta resolução e a sensibilidade à radiação ultravioleta do Hubble, permitiu aos astrônomos observarem nós brilhantes de estrelas azuis, quentes, se formando juntamente com jatos de buracos negros ativos encontrados nos centros das gigantescas galáxias elípticas.
Combinando dados do Huubble com observações feitas por um conjunto de telescópios baseados tanto em Terra como no espaço, duas equipes independentes descobriram que os jatos dos buracos negros, e as estrelas recém-nascidas são todos partes de um ciclo auto-regulado. Jatos de alta energia atirados do buraco negro aquecem um halo de gás circulante, controlando a taxa com a qual o gás esfria e cai na galáxia.
“Pense no gás ao redor da galáxia como uma atmosfera”, explicou o líder do primeiro estudo, Megan Donahue, da Universidade Estadual do Michigan. “Essa atmosfera pode conter material em diferentes estados, do mesmo modo que a nossa atmosfera tem gás, nuvens e chuva. O que nós estamos vendo é um processo parecido com uma tempestade. À medida que os jatos impulsionam o gás para fora do centro da galáxia, parte do gás esfria e precipita em aglomerados frios que caem de volta para o centro da galáxia como gotas de chuvas”.
“As gotas de chuva eventualmente esfriam o suficiente para tornar-se nuvens de formação de estrelas de gás frio molecular, e a capacidade de observar no ultravioleta distante do Hubble, nos permitiu observar diretamente esses chuviscos de formação de estrelas”, explicou o líder do segundo estudo, Grant Tremblay, da Universidade de Yale. “Nós sabemos que esses chuviscos estão linkados com os jatos, pois eles foram encontrados em filamentos que se dobram ao redor dos jatos, ou abraçam as bordas de bolhas gigantes que os jatos inflaram”, disse Tremblay. “E eles terminam fazendo um redemoinho de gás de formação de estrelas ao redor do buraco negro central”.
Stellar-mass black holes in the Hyades star cluster?Sérgio Sacani
Astrophysical models of binary-black hole mergers in the Universe require a significant fraction of stellar-mass black holes (BHs)
to receive negligible natal kicks to explain the gravitational wave detections. This implies that BHs should be retained even in
open clusters with low escape velocities (≲ 1 km/s). We search for signatures of the presence of BHs in the nearest open cluster
to the Sun – the Hyades – by comparing density profiles of direct 𝑁-body models to data from Gaia. The observations are best
reproduced by models with 2−3 BHs at present. Models that never possessed BHs have an half-mass radius ∼ 30% smaller than
the observed value, while those where the last BHs were ejected recently (≲ 150 Myr ago) can still reproduce the density profile.
In 50% of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at ∼ 103 yr, making
them unlikely to be found through velocity variations. We look for potential BH companions through large Gaia astrometric and
spectroscopic errors, identifying 56 binary candidates - none of which consistent with a massive compact companion. Models
with 2 − 3 BHs have an elevated central velocity dispersion, but observations can not yet discriminate. We conclude that the
present-day structure of the Hyades requires a significant fraction of BHs to receive natal kicks smaller than the escape velocity
of ∼ 3 km s−1
at the time of BH formation and that the nearest BHs to the Sun are in, or near, Hyades.
Disks of Stars in the Galactic Center Triggered by Tidal Disruption EventsSérgio Sacani
This document proposes that tidal disruption events (TDEs) from wandering stars could trigger episodes of positive star formation feedback in the Galactic Center, providing an explanation for the observed disks of young stars near Sgr A*. When a star is tidally disrupted by the supermassive black hole, the resulting jet compresses gas clouds to densities high enough to resist tidal forces and form stars within the disk plane perpendicular to the jet. The estimated rate of jetted TDEs is consistent with the age of the disk stars. This mechanism predicts a random orientation for each disk and the potential for multiple misaligned disks from separate TDE events.
Galaxy interactions are the dominant trigger for local type 2 quasarsSérgio Sacani
The triggering mechanism for the most luminous, quasar-like active galactic nuclei (AGN) remains a source of debate, with
some studies favouring triggering via galaxy mergers, but others finding little evidence to support this mechanism. Here, we
present deep Isaac Newton Telescope/Wide Field Camera imaging observations of a complete sample of 48 optically selected
type 2 quasars – the QSOFEED sample (L[O III] > 108.5 L; z < 0.14). Based on visual inspection by eight classifiers, we find
clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with
65+6
−7 per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared
with only 22+5
−4 per cent of a stellar-mass- and redshift-matched comparison sample of non-AGN galaxies – a 5σ difference. The
type 2 quasar hosts are a factor of 3.0+0.5 −0.8 more likely to be morphologically disturbed than their matched non-AGN counterparts,
similar to our previous results for powerful 3CR radio AGN of comparable [O III] emission-line luminosity and redshift. In
contrast to the idea that quasars are triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only become
visible post-coalescence, the majority of morphologically disturbed type 2 quasar sources in our sample are observed in the
pre-coalescence phase (61+8
−9 per cent). We argue that much of the apparent ambiguity that surrounds observational results in this
field is a result of differences in the surface brightness depths of the observations, combined with the effects of cosmological
surface brightness dimming.
We present deep optical images of the Large and Small Magellanic Clouds (LMC and SMC) using
a low cost telephoto lens with a wide field of view to explore stellar substructure in the outskirts
of the stellar disk of the LMC (r < 10 degrees from the center). These data have higher resolution
than existing star count maps, and highlight the existence of stellar arcs and multiple spiral arms in
the northern periphery, with no comparable counterparts in the South. We compare these data to
detailed simulations of the LMC disk outskirts, following interactions with its low mass companion,
the SMC. We consider interaction in isolation and with the inclusion of the Milky Way tidal field.
The simulations are used to assess the origin of the northern structures, including also the low density
stellar arc recently identified in the DES data by Mackey et al. (2015) at ∼ 15 degrees. We conclude
that repeated close interactions with the SMC are primarily responsible for the asymmetric stellar
structures seen in the periphery of the LMC. The orientation and density of these arcs can be used to
constrain the LMC’s interaction history with and impact parameter of the SMC. More generally, we
find that such asymmetric structures should be ubiquitous about pairs of dwarfs and can persist for
1-2 Gyr even after the secondary merges entirely with the primary. As such, the lack of a companion
around a Magellanic Irregular does not disprove the hypothesis that their asymmetric structures are
driven by dwarf-dwarf interactions.
Formation of low mass protostars and their circumstellar disksSérgio Sacani
Understanding circumstellar disks is of prime importance in astrophysics, however, their birth process remains poorly constrained due to observational and numerical challenges. Recent numerical works have shown that the small-scale physics, often wrapped into a sub-grid model, play a crucial role in disk formation and evolution. This calls for a combined approach in which both the protostar and circumstellar disk are studied in concert. Aims. We aim to elucidate the small scale physics and constrain sub-grid parameters commonly chosen in the literature by resolving the star-disk interaction. Methods. We carry out a set of very high resolution 3D radiative-hydrodynamics simulations that self-consistently describe the collapse of a turbulent dense molecular cloud core to stellar densities. We study the birth of the protostar, the circumstellar disk, and its early evolution (< 6 yr after protostellar formation). Results. Following the second gravitational collapse, the nascent protostar quickly reaches breakup velocity and sheds its surface material, thus forming a hot (∼ 103 K), dense, and highly flared circumstellar disk. The protostar is embedded within the disk, such that material can flow without crossing any shock fronts. The circumstellar disk mass quickly exceeds that of the protostar, and its kinematics are dominated by self-gravity. Accretion onto the disk is highly anisotropic, and accretion onto the protostar mainly occurs through material that slides on the disk surface. The polar mass flux is negligible in comparison. The radiative behavior also displays a strong anisotropy, as the polar accretion shock is shown to be supercritical whereas its equatorial counterpart is subcritical. We also f ind a remarkable convergence of our results with respect to initial conditions. Conclusions. These results reveal the structure and kinematics in the smallest spatial scales relevant to protostellar and circumstellar disk evolution. They can be used to describe accretion onto regions commonly described by sub-grid models in simulations studying larger scale physics.
DISCOVERY OF A GALAXY CLUSTER WITH A VIOLENTLY STARBURSTING CORE AT z = 2:506Sérgio Sacani
We report the discovery of a remarkable concentration of massive galaxies with extended X-ray
emission at zspec = 2:506, which contains 11 massive (M & 1011M) galaxies in the central 80kpc
region (11.6 overdensity). We have spectroscopically conrmed 17 member galaxies with 11 from CO
and the remaining ones from H. The X-ray luminosity, stellar mass content and velocity dispersion
all point to a collapsed, cluster-sized dark matter halo with mass M200c = 1013:90:2M, making it
the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this
structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive
galaxies classied as quiescent. The star formation rate (SFR) in the 80kpc core reaches 3400 M
yr 1 with a gas depletion time of 200 Myr, suggesting that we caught this cluster in rapid build-up
of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst
fraction ( 25%, compared to 3%-5% in the eld). The presence of both a collapsed, cluster-sized
halo and a predominant population of massive SFGs suggests that this structure could represent an
important transition phase between protoclusters and mature clusters. It provides evidence that the
main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster,
providing new insights into massive cluster formation at early epochs. The large integrated stellar
mass at such high redshift challenges our understanding of massive cluster formation.
Similar to HST imaging of star-forming clumps in 6 GASP ram-pressure stripped galaxies (20)
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these
estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo
resulting from a well-understood near-surface phenomenon improves prospects
for accurate predictions of full magnetic cycles and space weather, affecting the
electromagnetic infrastructure of Earth.
Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Mat...Sérgio Sacani
In the Nice model of solar system formation, Uranus and Neptune undergo an orbital upheaval,
sweeping through a planetesimal disk. The region of the disk from which material is accreted by
the ice giants during this phase of their evolution has not previously been identified. We perform
direct N-body orbital simulations of the four giant planets to determine the amount and origin of solid
accretion during this orbital upheaval. We find that the ice giants undergo an extreme bombardment
event, with collision rates as much as ∼3 per hour assuming km-sized planetesimals, increasing the
total planet mass by up to ∼0.35%. In all cases, the initially outermost ice giant experiences the
largest total enhancement. We determine that for some plausible planetesimal properties, the resulting
atmospheric enrichment could potentially produce sufficient latent heat to alter the planetary cooling
timescale according to existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric composition and
thermal evolution of the ice giants, motivating future work on the fate of deposited solid material.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for...Sérgio Sacani
Mars is a particularly attractive candidate among known astronomical objects
to potentially host life. Results from space exploration missions have provided
insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to
its toxicity. However, it can also provide potential benefits, such as producing
brines by deliquescence, like those thought to exist on present-day Mars. Here
we show perchlorate brines support folding and catalysis of functional RNAs,
while inactivating representative protein enzymes. Additionally, we show
perchlorate and other oxychlorine species enable ribozyme functions,
including homeostasis-like regulatory behavior and ribozyme-catalyzed
chlorination of organic molecules. We suggest nucleic acids are uniquely wellsuited to hypersaline Martian environments. Furthermore, Martian near- or
subsurface oxychlorine brines, and brines found in potential lifeforms, could
provide a unique niche for biomolecular evolution.
Continuum emission from within the plunging region of black hole discsSérgio Sacani
The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a
powerful probe of the mass and spin of the central black hole. The vast majority of existing ‘continuum fitting’ models neglect
emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however,
find non-zero emission sourced from these regions. In this work, we extend existing techniques by including the emission
sourced from within the plunging region, utilizing new analytical models that reproduce the properties of numerical accretion
simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component,
but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component
has been added in by hand in an ad hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum
of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional
models that neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of
intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820 + 070 black hole spin
which must be low a• < 0.5 to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission
component in the MAXI J1820 + 070 spectrum between 6 and 10 keV, highlighting the necessity of including this region. Our
continuum fitting model is made publicly available.
WASP-69b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 RpSérgio Sacani
Studying the escaping atmospheres of highly irradiated exoplanets is critical for understanding the physical
mechanisms that shape the demographics of close-in planets. A number of planetary outflows have been observed
as excess H/He absorption during/after transit. Such an outflow has been observed for WASP-69b by multiple
groups that disagree on the geometry and velocity structure of the outflow. Here, we report the detection of this
planet’s outflow using Keck/NIRSPEC for the first time. We observed the outflow 1.28 hr after egress until the
target set, demonstrating the outflow extends at least 5.8 × 105 km or 7.5 Rp This detection is significantly longer
than previous observations, which report an outflow extending ∼2.2 planet radii just 1 yr prior. The outflow is
blueshifted by −23 km s−1 in the planetary rest frame. We estimate a current mass-loss rate of 1 M⊕ Gyr−1
. Our
observations are most consistent with an outflow that is strongly sculpted by ram pressure from the stellar wind.
However, potential variability in the outflow could be due to time-varying interactions with the stellar wind or
differences in instrumental precision.
Immersive Learning That Works: Research Grounding and Paths ForwardLeonel Morgado
We will metaverse into the essence of immersive learning, into its three dimensions and conceptual models. This approach encompasses elements from teaching methodologies to social involvement, through organizational concerns and technologies. Challenging the perception of learning as knowledge transfer, we introduce a 'Uses, Practices & Strategies' model operationalized by the 'Immersive Learning Brain' and ‘Immersion Cube’ frameworks. This approach offers a comprehensive guide through the intricacies of immersive educational experiences and spotlighting research frontiers, along the immersion dimensions of system, narrative, and agency. Our discourse extends to stakeholders beyond the academic sphere, addressing the interests of technologists, instructional designers, and policymakers. We span various contexts, from formal education to organizational transformation to the new horizon of an AI-pervasive society. This keynote aims to unite the iLRN community in a collaborative journey towards a future where immersive learning research and practice coalesce, paving the way for innovative educational research and practice landscapes.
hematic appreciation test is a psychological assessment tool used to measure an individual's appreciation and understanding of specific themes or topics. This test helps to evaluate an individual's ability to connect different ideas and concepts within a given theme, as well as their overall comprehension and interpretation skills. The results of the test can provide valuable insights into an individual's cognitive abilities, creativity, and critical thinking skills
Unlocking the mysteries of reproduction: Exploring fecundity and gonadosomati...AbdullaAlAsif1
The pygmy halfbeak Dermogenys colletei, is known for its viviparous nature, this presents an intriguing case of relatively low fecundity, raising questions about potential compensatory reproductive strategies employed by this species. Our study delves into the examination of fecundity and the Gonadosomatic Index (GSI) in the Pygmy Halfbeak, D. colletei (Meisner, 2001), an intriguing viviparous fish indigenous to Sarawak, Borneo. We hypothesize that the Pygmy halfbeak, D. colletei, may exhibit unique reproductive adaptations to offset its low fecundity, thus enhancing its survival and fitness. To address this, we conducted a comprehensive study utilizing 28 mature female specimens of D. colletei, carefully measuring fecundity and GSI to shed light on the reproductive adaptations of this species. Our findings reveal that D. colletei indeed exhibits low fecundity, with a mean of 16.76 ± 2.01, and a mean GSI of 12.83 ± 1.27, providing crucial insights into the reproductive mechanisms at play in this species. These results underscore the existence of unique reproductive strategies in D. colletei, enabling its adaptation and persistence in Borneo's diverse aquatic ecosystems, and call for further ecological research to elucidate these mechanisms. This study lends to a better understanding of viviparous fish in Borneo and contributes to the broader field of aquatic ecology, enhancing our knowledge of species adaptations to unique ecological challenges.
ESA/ACT Science Coffee: Diego Blas - Gravitational wave detection with orbita...Advanced-Concepts-Team
Presentation in the Science Coffee of the Advanced Concepts Team of the European Space Agency on the 07.06.2024.
Speaker: Diego Blas (IFAE/ICREA)
Title: Gravitational wave detection with orbital motion of Moon and artificial
Abstract:
In this talk I will describe some recent ideas to find gravitational waves from supermassive black holes or of primordial origin by studying their secular effect on the orbital motion of the Moon or satellites that are laser ranged.
Phenomics assisted breeding in crop improvementIshaGoswami9
As the population is increasing and will reach about 9 billion upto 2050. Also due to climate change, it is difficult to meet the food requirement of such a large population. Facing the challenges presented by resource shortages, climate
change, and increasing global population, crop yield and quality need to be improved in a sustainable way over the coming decades. Genetic improvement by breeding is the best way to increase crop productivity. With the rapid progression of functional
genomics, an increasing number of crop genomes have been sequenced and dozens of genes influencing key agronomic traits have been identified. However, current genome sequence information has not been adequately exploited for understanding
the complex characteristics of multiple gene, owing to a lack of crop phenotypic data. Efficient, automatic, and accurate technologies and platforms that can capture phenotypic data that can
be linked to genomics information for crop improvement at all growth stages have become as important as genotyping. Thus,
high-throughput phenotyping has become the major bottleneck restricting crop breeding. Plant phenomics has been defined as the high-throughput, accurate acquisition and analysis of multi-dimensional phenotypes
during crop growing stages at the organism level, including the cell, tissue, organ, individual plant, plot, and field levels. With the rapid development of novel sensors, imaging technology,
and analysis methods, numerous infrastructure platforms have been developed for phenotyping.
Or: Beyond linear.
Abstract: Equivariant neural networks are neural networks that incorporate symmetries. The nonlinear activation functions in these networks result in interesting nonlinear equivariant maps between simple representations, and motivate the key player of this talk: piecewise linear representation theory.
Disclaimer: No one is perfect, so please mind that there might be mistakes and typos.
dtubbenhauer@gmail.com
Corrected slides: dtubbenhauer.com/talks.html
PPT on Direct Seeded Rice presented at the three-day 'Training and Validation Workshop on Modules of Climate Smart Agriculture (CSA) Technologies in South Asia' workshop on April 22, 2024.
ESR spectroscopy in liquid food and beverages.pptxPRIYANKA PATEL
With increasing population, people need to rely on packaged food stuffs. Packaging of food materials requires the preservation of food. There are various methods for the treatment of food to preserve them and irradiation treatment of food is one of them. It is the most common and the most harmless method for the food preservation as it does not alter the necessary micronutrients of food materials. Although irradiated food doesn’t cause any harm to the human health but still the quality assessment of food is required to provide consumers with necessary information about the food. ESR spectroscopy is the most sophisticated way to investigate the quality of the food and the free radicals induced during the processing of the food. ESR spin trapping technique is useful for the detection of highly unstable radicals in the food. The antioxidant capability of liquid food and beverages in mainly performed by spin trapping technique.
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and the revised UWWTD (Urban Waste Water Treatment Directive)”
Describing and Interpreting an Immersive Learning Case with the Immersion Cub...Leonel Morgado
Current descriptions of immersive learning cases are often difficult or impossible to compare. This is due to a myriad of different options on what details to include, which aspects are relevant, and on the descriptive approaches employed. Also, these aspects often combine very specific details with more general guidelines or indicate intents and rationales without clarifying their implementation. In this paper we provide a method to describe immersive learning cases that is structured to enable comparisons, yet flexible enough to allow researchers and practitioners to decide which aspects to include. This method leverages a taxonomy that classifies educational aspects at three levels (uses, practices, and strategies) and then utilizes two frameworks, the Immersive Learning Brain and the Immersion Cube, to enable a structured description and interpretation of immersive learning cases. The method is then demonstrated on a published immersive learning case on training for wind turbine maintenance using virtual reality. Applying the method results in a structured artifact, the Immersive Learning Case Sheet, that tags the case with its proximal uses, practices, and strategies, and refines the free text case description to ensure that matching details are included. This contribution is thus a case description method in support of future comparative research of immersive learning cases. We then discuss how the resulting description and interpretation can be leveraged to change immersion learning cases, by enriching them (considering low-effort changes or additions) or innovating (exploring more challenging avenues of transformation). The method holds significant promise to support better-grounded research in immersive learning.
The technology uses reclaimed CO₂ as the dyeing medium in a closed loop process. When pressurized, CO₂ becomes supercritical (SC-CO₂). In this state CO₂ has a very high solvent power, allowing the dye to dissolve easily.
HST imaging of star-forming clumps in 6 GASP ram-pressure stripped galaxies
1. Draft version February 22, 2023
Typeset using L
A
TEX twocolumn style in AASTeX631
HST imaging of star-forming clumps in 6 GASP ram-pressure stripped galaxies
Eric Giunchi ,1, 2
Marco Gullieuszik ,1
Bianca M. Poggianti ,1
Alessia Moretti ,1
Ariel Werle ,1
Claudia Scarlata ,3
Anita Zanella ,1
Benedetta Vulcani ,1
and Daniela Calzetti 4
1INAF-Osservatorio astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
2Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo Osservatorio 3, 35122 Padova, Italy
3Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 316 Church Street SE, Minneapolis,
MN 55455, USA
4Department of Astronomy, University of Massachusetts, 710 N. Pleasant Street, LGRT 619J, Amherst, MA 01002, USA
ABSTRACT
Exploiting broad- and narrow-band images of the Hubble Space Telescope from near-UV to I-band
restframe, we study the star-forming clumps of six galaxies of the GASP sample undergoing strong
ram-pressure stripping (RPS). Clumps are detected in Hα and near-UV, tracing star formation on
different timescales. We consider clumps located in galaxy disks, in the stripped tails and those
formed in stripped gas but still close to the disk, called extraplanar. We detect 2406 Hα-selected
clumps (1708 in disks, 375 in extraplanar regions, and 323 in tails) and 3750 UV-selected clumps (2026
disk clumps, 825 extraplanar clumps and 899 tail clumps). Only ∼ 15% of star-forming clumps are
spatially resolved, meaning that most are smaller than ∼ 140 pc. We study the luminosity and size
distribution functions (LDFs and SDFs, respectively) and the luminosity-size relation. The average
LDF slope is 1.79 ± 0.09, while the average SDF slope is 3.1 ± 0.5. Results suggest the star formation
to be turbulence driven and scale-free, as in main-sequence galaxies. All the clumps, whether they are
in the disks or in the tails, have an enhanced Hα luminosity at a given size, compared to the clumps in
main-sequence galaxies. Indeed, their Hα luminosity is closer to that of clumps in starburst galaxies,
indicating that ram pressure is able to enhance the luminosity. No striking differences are found among
disk and tail clumps, suggesting that the different environments in which they are embedded play a
minor role in influencing the star formation.
Keywords: galaxies: clusters - galaxies: evolution – galaxies: peculiar – galaxies: star formation –
galaxies: structure
1. INTRODUCTION
Star formation is the mechanism driving the condensa-
tion of atomic gas from galactic to sub-kpc scales down
to the dense cores (on pc/sub-pc scales) in which stars
eventually form (Section 4 in Kennicutt & Evans 2012)
and therefore studying which processes are capable of
influencing it is fundamental to our understanding of
galaxy formation and evolution. The bridge between the
galactic- and core-scale regimes is represented by & 10
pc-scaled star-forming clumps, with masses & 104
M
(Portegies Zwart et al. 2010). Our knowledge about
these structures has greatly improved in the last decade
thanks to observational surveys of low redshift galaxies
with the Hubble Space Telescope (HST), which is able to
achieve the resolution necessary to study their morphol-
ogy and size properties (LEGUS, Calzetti et al. 2015;
DYNAMO, Fisher et al. 2017; LARS, Messa et al. 2019;
PHANGS-HST; Lee et al. 2020). Exploiting LEGUS
data, many studies find hints that star formation is a
turbulence-driven process fragmenting the gas following
a scale-free hierarchy (Elmegreen et al. 2014; Gouliermis
et al. 2015; Gouliermis et al. 2017). The hierarchical
structure is then reflected also in the emerging spatial
distribution of stars formed from such gas (Elmegreen
& Falgarone 1996; Elmegreen 2006; Grasha et al. 2017).
Moreover, the star formation mechanism can be
strongly influenced by the properties of the local
medium in which the star forming clumps form, and
this leaves an imprint that can be studied using differ-
ent diagnostics.
Models describing the fragmentation of star-forming
regions as a scale-free, turbulence-driven process pre-
dict the mass distribution function of these regions to
be a power law with slope 2 (Elmegreen 2006). The
arXiv:2302.10615v1
[astro-ph.GA]
21
Feb
2023
2. 2 GASP team
corresponding luminosity distribution function (LDF),
if derived using a tracer of a narrow age range, is ex-
pected to have a similar slope, provided that 1) the
Initial Mass Function (IMF) is well sampled and in-
dependent of the mass of the initial cloud from which
the clumps are formed and 2) the star formation his-
tory (SFH) and therefore the stellar age distributions of
all clouds are the same (Elmegreen & Falgarone 1996;
Elmegreen 2006).
Indeed, from the observational point of view, LDFs
of recently formed star-forming regions are known to be
well described by a power law, independently of wave-
length, with a drop at luminosities fainter than a peak
luminosity Lpeak due to incompleteness (e.g., Hα: Ken-
nicutt et al. 1989; Santoro et al. 2022; UV Cook et al.
2016; Messa et al. 2019; V-band Larsen 2002; Bastian
et al. 2007, R-band Whitmore et al. 2014; IR and radio
Mascoop et al. 2021). In general, observed slopes are
found to be consistent with 2, even though in some cases
there are hints of a slope slightly smaller than 2 (Cook
et al. 2016: 1.76 ± 0.3; Santoro et al. 2022: 1.73 ± 0.15).
Interestingly, some of these works find the value of the
slope to be affected by the local environment. Cook
et al. (2016) and Santoro et al. (2022) show that the
LDF flattens in regions with high star-formation rate
(SFR) surface density (ΣSFR). The same trend is then
reflected in the LDF of clumps belonging to different in-
tergalactic environments: Messa et al. (2018) show that
the LDF of UV clumps in the spiral arms of the LE-
GUS galaxy M51 is flatter than that of clumps in the
inter-arm region (with likely lower ΣSFR).
The size distribution function (SDF) of star-forming
clumps is known to be well described by a power law
as well (Kennicutt & Hodge 1980; Gusev 2014), with
slopes between 2.5 and 4.5, and a flatter distribution for
an increasing level of clustering in the clumps.
The luminosity (LHα typically)-size relation, which
many works (Wisnioski et al. 2012; Cosens et al. 2018
and references therein) have shown to be a linear rela-
tion in the logarithmic plane, is another proxy of the
properties of star formation. The slope and the normal-
ization of the correlation are thought to be related to
the geometrical properties of the HII regions ionized by
young stars and to the star-formation rate surface den-
sity ΣSFR. As shown in Cosens et al. (2018), clumps in
starburst-like environments, therefore with a high ΣSFR,
are likely to have a higher Hα luminosity at a given size
and to follow a flatter distribution.
Jellyfish galaxies are a great laboratory to study star
formation in peculiar regimes and environments. Jel-
lyfish galaxies are cluster galaxies undergoing strong
ram-pressure stripping (RPS, Gunn & Gott 1972). The
ram pressure exerted by the intracluster medium (ICM)
is able to strip the gas from the galactic interstellar
medium (ISM), eventually producing tails up to more
than 100 kpc long, but leaving the stellar disk almost
undisturbed (Poggianti et al. 2017a). The gas removal
accelerates the quenching of the star formation in the
galaxy (Cortese et al. 2021). Several previous works
find RPS to be able to briefly enhance star formation
during the first stages of the stripping process and to
trigger in-situ star formation in compact knots of gas
stripped out of the galactic disks (Yoshida et al. 2008;
Smith et al. 2010; Merluzzi et al. 2013; Fossati et al.
2016; Consolandi et al. 2017; Jáchym et al. 2019). The
tails of these galaxies give the unprecedented opportu-
nity to study the star-formation mechanism in a hotter
and higher-pressure environment than the galactic ISM
and without the influence of the underlying contamina-
tion of the old stellar populations in the disk.
Abramson & Kenney (2014) and Kenney et al. (2015)
observed galaxies undergoing RPS in the Virgo cluster
(NGC 4402 and NGC 4522) and in the Coma cluster
(NGC 4921) with HST, respectively. These works show
that RPS is able to decouple the high-density compo-
nent of the ISM (and in particular the Giant Molecular
Clouds) from the low-density one, which is more prone
to stripping. Also, dust is characterized by elongated
morphology and filaments aligned with the stripping di-
rection. Cramer et al. (2019) studied the long and nar-
row Hα tail of D100 in the Coma cluster with HST and
found unbound young UV sources with sizes ∼ 50 − 100
pc, which they consider likely to disperse with ageing.
One of the aims of the GASP (GAs Stripping Phenom-
ena in galaxies with MUSE, Poggianti et al. 2017a) ESO
Large Program is to study the properties of galaxies af-
fected by different gas removal processes in the field,
in groups and clusters. This includes cluster galaxies
in different RPS stages, from pre-stripping (the control
sample) to post-stripping (Fritz et al. 2017). Targets
were chosen from the catalog in Poggianti et al. (2016)
as galaxies with long unilateral debris in optical images,
suggestive of gas-only removal. The final sample in-
cludes galaxies in the mass range 109
− 1011.5
M and
at redshift 0.04 < z < 0.07. Targets were observed with
MUSE on the VLT (details about the MUSE data are
given in Sec. 2.2), in order to investigate the properties
of both the ionized gas phase and the stellar component
in the disks and in the stripped tails. Results of GASP
confirmed the presence of clumps with in-situ star for-
mation in the tails of individual jellyfish galaxies of the
sample (Bellhouse et al. 2017; Gullieuszik et al. 2017;
Moretti et al. 2018; Moretti et al. 2020; Poggianti et al.
2019). In particular, Poggianti et al. (2019) analyzed
3. AASTeX v6.3.1 Sample article 3
star-forming clumps in the tails and in the disks of 16
GASP galaxies, finding that tail clumps are less mas-
sive, both in terms of stellar and gas mass, and with a
smaller gas velocity dispersion than the clumps in the
disks. However, the spatial resolution of MUSE at the
typical redshift of the GASP galaxies (∼ 1 kpc) did not
allow us to study the morphology and the size of these
clumps.
In order to better characterize the properties of the
star-forming clumps detected in GASP galaxies with
MUSE, six galaxies of the GASP sample have been ob-
served with HST, whose resolution is about a factor 14
better than MUSE one (details in Sec. 2.1). The broad-
band filters which were adopted are the F275W, F336W,
F606W and F814W, covering a spectral range going
from UV- to I-band restframe. In addition, galaxies
were observed also with the narrow-band filter F680N,
collecting the Hα emission.
This paper is structured as follows: in Sec. 2 we
present the HST and MUSE data; in Sec. 3 we de-
fine the spatial categories; Sec. 4 is focused on the steps
followed for detecting and selecting star-forming clumps
and complexes; in Sec. 5 we present the samples of
clumps and complexes (5); in Sec. 6 we study the lu-
minosity and size distribution functions of clumps and
complexes (Secs. 6.1 and 6.2, respectively); Sec. 7 is
dedicated to the luminosity-size relation of the clumps;
in Sec. 8 the catalogs which will be publicly available
are described; in Sec. 9 we summarize our results.
This paper adopts standard concordance cosmology
parameters H0 = 70 km s−1
Mpc−1
, ΩM = 0.3 and ΩΛ =
0.7 and a Chabrier (2003) Initial Mass Function (IMF).
2. DATA
2.1. HST data
In this work, we focus on the study of the luminosities
and sizes of star-forming clumps and complexes in a sub-
sample of 6 GASP galaxies, whose main properties are
listed in Table 1. The galaxies were selected from the
GASP sample of ram-pressure stripped galaxies (Pog-
gianti et al. 2017b) for their extended Hα emitting tails
and, in particular, the large number of Hα clumps de-
tected with MUSE observations (Poggianti et al. 2019,
see also Vulcani et al. (2020)).
The galaxies were observed using the WFC3/UVIS
on board of the Hubble Space Telescope (HST), using
4 broad-band filters (F275W, F336W, F606W, F814W),
which cover a spectral range from UV- to I-band rest-
frame. In addition, galaxies were also observed with a
narrow-band filter, F680N, in order to collect the Hα
emission at the redshift of these galaxies. Details about
observations, data reduction, calibration and analysis,
estimate of the standard deviation of the background
in each band (σ, hereafter) and Hα + [NII] extraction
from the F680N band are described in Gullieuszik et al.
(2023), but here we summarise the most important prop-
erties.
All images collected have pixel angular size of 0.0400
.
The UVIS PSFs in all the 5 filters do not change sig-
nificantly and have a FWHM of 0.07001
, corresponding
to ∼ 70 pc at the redshifts of the clusters hosting these
galaxies (0.0424 − 0.0650, see Table 1). Images were
reduced and calibrated using AstroDrizzle2
. To ob-
tain the Hα + [NII] maps, the continuum emission is
modelled by linearly interpolating the emission coming
from the broad-band filters F606W and F814W and then
subtracted to the F680N images. Contamination from
emission lines within the two broad-band filters is ex-
pected to have only a small (< 10%) impact on our Hα
intensity estimates (see Gullieuszik et al. 2023).
For our purposes, in some cases we are going to work
also with denoised versions of these HST images. De-
noising is performed using a Python software package
called PySAP3
(Farrens et al. 2020). This algorithm
expands the image in Fourier series and we set its pa-
rameters in order to remove the high-frequency compo-
nents, which are typically due to noise. We removed the
component with the highest frequency, equal to 2 pixels.
This procedure allows us to detect also fainter regions
without being dominated by noise, but does not yield
reliable sizes. This is the reason for not working with
denoised images alone.
Throughout this paper, we work on a smaller squared
field-of-view (see Table 2) with respect to the entire
HST-WFC3/UVIS images (2.670
× 2.670
), still sufficient
to cover the entire extension of the galaxies and their
tails.
2.2. MUSE data
Throughout this work, we also exploit the informa-
tion obtained from the GASP survey to remove regions
powered by AGN or shocks using BPT maps (Bald-
win et al. 1981), confirm the redshift of star-forming
clump candidates and correct the F680N filter for the
line emission of NII. All galaxies were observed in ser-
vice mode with the Multi Unit Spectroscopic Explorer
(MUSE, Bacon et al. 2010). MUSE is an integral-field-
1 https://hst-docs.stsci.edu/wfc3ihb/chapter-6-uvis-imageing-
with-wfc3/6-6-uvis-optical-performance
2 https://drizzlepac.readthedocs.io/en/latest/CHANGELOG.html
3 https://cea-cosmic.github.io/pysap/index.html
4. 4 GASP team
Table 1. Summary of the main properties of the galaxies studied in this paper and of their host clusters.
IDP 16 RA Dec M∗
Hα
Hα+[NII]
zgal cluster σclus zclus ref
(J2000) (J2000) 1010
M km/s
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
JO175 20:51:17.593 -52:49:22.34 3.23.7
2.7 0.705 0.0468 A3716 753+36
−38 0.0457 (4,10,14)
JO201 00:41:30.295 -09:15:45.98 6.27.0
2.4 0.660 0.0446 A85 859+42
−44 0.0559
(3,4,5,6,8,9,10,
13,14,15,16,18)
JO204 10:13:46.842 -00:54:51.27 4.14.7
3.5 0.660 0.0424 A957 631+43
−40 0.0451 (2,4,6,12,18)
JO206 21:13:47.410 +02:28:35.50 9.110.0
8.2 0.703 0.0513 IIZW108 575+33
−31 0.0486 (1,4,6,10,13,18,19)
JW39 13:04:07.719 +19:12:38.41 1720
14 0.650 0.0650 A1668 654 0.0634 (14,18,19,21)
JW100 23:36:25.054 +21:09:02.64 2936
22 0.530 0.0602 A2626 650+53
−49 0.0548
(4,6,7,10,11,
15,17,18,19,20)
Note—Columns are: GASP ID of the galaxy as in Poggianti et al. 2016 (1), RA and Dec of the galaxy (2 and 3), galaxy
stellar mass (4), median value for Hα/(Hα + [NII]) from the MUSE clumps listed in Poggianti et al. (2019) (5), galaxy
redshift (6), ID of the host cluster (7), cluster velocity dispersion (8), cluster redshift (9), references (10). References
are: 1) Poggianti et al. (2017a), 2) Gullieuszik et al. (2017), 3) Bellhouse et al. (2017), 4) Poggianti et al. (2017b), 5)
George et al. (2018), 6) Moretti et al. (2018), 7) Poggianti et al. (2019), 8) Bellhouse et al. (2019), 9) George et al.
(2019), 10) Radovich et al. (2019), 11) Moretti et al. (2020), 12) Deb et al. (2020), 13) Ramatsoku et al. (2020), 14)
Bellhouse et al. (2021), 15) Tomičić et al. (2021a), 16) Campitiello et al. (2021), 17) Ignesti et al. (2022a), 18) Tomičić
et al. (2021b), 19) Ignesti et al. (2022b), 20) Sun et al. (2021), 21) Peluso et al. (2022). Masses are taken from Vulcani
et al. (2018). Cluster redshifts and velocity dispersions are taken from Biviano et al. (2017) and Cava et al. (2009).
Table 2. Properties of the HST images sub-FOV used to
detect clumps. For each galaxy (IDP 16) the center coordi-
nates (RAcenter and Deccenter) and the width of the sub-FOV
(width) are listed. Sub-FOV are always squared.
IDP 16 RAcenter Deccenter width
JO175 20:51:17.6169 -52:49:47.189 2.00’
JO201 0:41:31.7471 -9:16:02.613 2.67’
JO204 10:13:47.2256 -0:54:46.617 1.60’
JO206 21:13:44.1417 +2:28:06.535 3.27’
JW39 13:04:08.8132 +19:12:17.151 2.20’
JW100 23:36:23.0832 +21:04:47.508 1.93’
unit spectrograph with a 10
× 10
field of view, sampled
with 0.200
×0.200
pixels. The typical seeing of the MUSE
observations is 100
(0.7 − 1.3 kpc at the redshifts of these
galaxies, Table 1). Furthermore, MUSE spectra cover a
spectral range going from 4500 to 9300 Å, sampled at
∼ 1.25 Å/pixel and with a spectral resolution of 2.6 Å.
The data were reduced using the most recent version of
the MUSE pipeline available at the time of each obser-
vation (Bacon et al. 2010, from version 1.2 to 1.6), as
described in details in Poggianti et al. (2017a). The dat-
acubes were then corrected for Galactic extinction using
the extinction law and the reddending map by Cardelli
et al. (1989) and Schlegel et al. (1998) (considering the
recalibration introduced by Schlafly & Finkbeiner 2011),
respectively. Fluxes, velocities and velocity dispersions
of the gas emission lines were obtained using KUBEVIZ
(Fossati et al. 2016), after subtracting the stellar-only
component derived with SINOPSIS (Fritz et al. 2017).
3. DEFINITION OF DISK, EXTRAPLANAR AND
TAIL REGIONS
Throughout this work, we are interested in studying
the effects of local environment on star formation, and
thus we aim at distinguishing star-forming regions orig-
inating from stripped gas embedded in the cluster envi-
ronment from those still in the galaxy disk.
In analogy with what was done for the MUSE obser-
vations (Gullieuszik et al. 2020), the starting point to
define the stripped tails is the definition of the galaxy
stellar disk. As already noted in Gullieuszik et al. (2023)
(see also Fig. 1), the high spatial resolution of HST al-
lows us to characterize the galaxy substructures and the
stellar disk in more detail than what is possible with
MUSE. We used the 2σ contour of the reddest photo-
metric band available (F814W) to draw the most exter-
nal boundary of the stellar optical disk. 2σ values range
from 2.14 to 2.67 × 10−21
erg/s/cm2
/Å per pixel. We
will refer to this contour as the galaxy optical contour
5. AASTeX v6.3.1 Sample article 5
(white dashed lines in Fig. 1) and we define as tail the
region beyond it.
In the disks of these galaxies there are some regions
particularly bright in UV (band F275W), faint in opti-
cal (band F814W), elongated and aligned in the same
direction of the tails. Therefore, they are likely to be
young stellar populations formed in gas already stripped
by ram pressure, but still inside the galaxy optical con-
tour because of projection effects or because RPS is at
an early stage: we call these regions extraplanar. In or-
der to separate the extraplanar regions from those still
in the disk, we visually inspected the RGB images and
traced an inner disk contour (red solid lines in Fig. 1),
using the UV contours of the images as a guide (green
contours in Fig. 1) to separate clumps with an elongated
appearance and aligned along the likely stripping direc-
tion from those who do not. We define as disk the region
within the inner disk contour and as extraplanar the re-
gion within the galaxy optical contour but outside the
inner disk contour. We point out that this process can-
not completely separate undisturbed and stripped gas,
since it is done via visual inspection and projection ef-
fects may prevent a perfect separation of these two cat-
egories of gas.
4. CLUMPS AND COMPLEXES DETECTION
This section presents the procedure we developed to
detect star-forming clumps and to measure their proper-
ties. This procedure was applied independently to both
the F275W and Hα images (Sec. 4.2), in order to trace
star formation on different timescales (∼ 200 Myr and
∼ 10 Myr, respectively, Kennicutt 1998; Kennicutt &
Evans 2012; Haydon et al. 2020). In addition, a differ-
ent version of it is applied to the F606W images to fully
recover the stellar content in the galaxy tails (Sec. 4.3).
4.1. Preliminary steps and Astrodendro
performance
As a first step, foreground and background sources
are masked out. This is done using, when available, the
spectroscopic information from MUSE and by visually
inspecting the RGB images constructed as described in
(Gullieuszik et al. 2023), looking for red elliptical or blue
spiral-armed sources, likely to be early-type and spiral
galaxies, respectively.
The clump detection is performed using Astroden-
dro4
, a software package created to compute dendro-
grams of observed or simulated Astronomical data, clas-
sifying them in a hierarchical tree structure. With this
software we are able to detect not only bright clumps,
4 http://www.dendrograms.org/
but also sub-clumps inside them. Fig. 2 shows an illus-
tration of 3 possible structures that Astrodendro can
generate.
Clumps are defined as local maxima on the image;
then the image is analyzed at fainter and fainter flux
levels and the clumps grow by including fainter pixels.
Eventually, at some point, adjacent clumps might blend
together. In this case, those clumps stop growing and
are defined as children of a common parent clump; for
the following steps, when fainter flux levels are consid-
ered, only the parent clump keeps growing. When the
flux threshold reaches a given value (see min value in
Appendix A), the algorithm stops and the tree structure
is built: starting from the clumps at the base of the tree
(i.e. the most extended ones), to which a level equal to
0 is assigned. Astrodendro retraces the tree and as-
signs to the sub-clumps a level equal to the level of their
parent clump +1. It also generates a mask to define all
pixels corresponding to each clump.
The naming convention used to define the position of
the clump in the tree hierarchy is as follows:
• trunk: clump with level = 0, regardless of whether
it contains sub-clumps or not;
• branch: clump with level > 0 and parent of other
clumps;
• leaf : clump with no children sub-clumps. Notice
that, according to this definition, a trunk can be
also a leaf.
4.1.1. Observed properties
For all the detected clumps, the following quantities
are computed by Astrodendro:
• the intensity-weighted mean position of the clump
in the plane of the sky, hereafter adopted as clump
center;
• semi-major and semi-minor axes computed as
standard deviations of the flux distribution of the
clump in the direction of greatest elongation in the
plane of the sky;
• the radius rcore computed as the geometric mean
of the major and minor axes;
• the exact area of the clump on the plane of the sky
A.
In addition, we computed the following quantities:
• the flux densities for all the photometric bands,
integrated over the clump area A. The flux un-
certainties are computed summing two contributes
6. 6 GASP team
Figure 1. Zoomed-in RGB images of JO175, JO201, JO204, JO206, JW39, JW100 (from top left to bottom right). The three
colours of the RGB images are: F814W (red), F606W (green), a combination of F275W, F606W and F814W (blue). Details
are given in Gullieuszik et al. 2023. The white dashed contours are the optical disks, defined as the 2σ contour in F814W. The
green dashed contours are the 1σ, 2σ, 3σ and 5σ UV (F275W+F336W) contours (smoothed for clarity). The red line is the
inner disk contour, traced as described in Sect. 3.
Leaf
Leaf Leaf
Leaf
Leaf
Leaf
0
2
1
LEVEL
Trunk
Trunk
Branch
A B
C
Figure 2. Illustration of the dendrogram structures built
by Astrodendro. Each clump is labelled with its position
(trunk, branch and leaf) and colored according to its level in
the tree hierarchy (from 0 to 2).
in quadrature: the background noise, computed
as a function of the clump area as described in
Gullieuszik et al. (2023), and the Poissonian un-
certainty on the source counts converted then
into flux considering the conversion factor PHOT-
FLAM, the exposure time and the Milky-Way dust
attenuation;
• Luminosity: calculated from the flux densities us-
ing the redshift of the cluster hosting the galaxy
(column 8 in Table 1). In order to get Hα lumi-
nosities, we compute Hα/(Hα + [NII]) for the Hα
clumps detected with MUSE in the galaxies of our
sample (Poggianti et al. 2019). The median val-
ues obtained for each galaxies are listed in Tab. 1
and used to correct the F680Nline flux for the NII
emission lines;
• rcore,corr: PSF-corrected core radius. It is com-
puted by subtracting in quadrature the σ of the
PSF (FWHM/2.35 ' 0.0300
, see Sec. 2.1) to rcore
and it is converted in physical scale according to
the redshift of the hosting cluster of each galaxy;
• the isophotal radius, defined as
7. AASTeX v6.3.1 Sample article 7
Figure 3. Comparison between the isophotal radius (riso)
and twice the PSF-corrected core radius (2rcore,corr), defined
in Sec. 4.1.1, both for Hα-resolved clumps (left panel) and
UV-resolved clumps (right panel). The grey dashed line is
the 1 : 1 relation, while the red solid line is the best-fitting
line. The best-fitting line is in good agreement with the 1 : 1
relation, both for Hα- and UV-resolved clumps.
riso =
r
A
π
. (1)
• size: defined as 2rcore,corr. This choice is supported
by the fact that rcore,corr is defined by the flux dis-
tribution of the clump, therefore it is less sensitive
to the flux threshold above which clumps are de-
tected (Wisnioski et al. 2012). Similarly to Wis-
nioski et al. (2012), in Fig. 3 we plot twice the
PSF-corrected core radius against the isophotal
radius, to show that these two quantities almost
follow a 1:1 relation.5
4.2. Star-forming clumps
5 In our size range, also Wisnioski et al. (2012) found that their
isophotal radii are larger than core radii by a factor ∼ 2.
Star-forming clumps are identified in the F275W (UV-
selected clumps) and Hα (Hα-selected clumps) images
running Astrodendro with a flux threshold of 2.5σ
on the original images and 2σ on the denoised images6
.
The two samples are computed independently, meaning
that, in principle, some UV- and Hα-selected clumps
may overlap if the same region is bright enough in both
the filters. Details about the parameters set for Astro-
dendro and the methods can be found in Appendix
A. Throughout the paper, we use only leaf and trunk
clumps (LT sample), unless otherwise stated, to avoid
considering the same region too many times.
Astrodendro detected an initial total of 6090 Hα
and 6259 UV candidates. To minimize the number of
spurious detections we adopted the following procedure,
which is schematised in the flow chart shown in Fig. 4.
Firstly, for each of the 5 photometric bands, we flagged
a clump as detected if its flux has a signal-to-noise ratio
SNR7
higher than 2. We then exclude all clumps that
were not detected in at least 3 photometric bands or in
both F275W and F680N8
. These criteria yield a reliable
detection of clumps, as confirmed by subsequent visual
inspection. A total 3611 Hα and 2293 UV spurious de-
tections were removed. As an example, in Fig 5 we show
the images in the 5 filters and in Hα of four Hα-selected
clump candidates of JO201: the first one (upper left
panel) is clearly detected in all images; the second one
(upper right panel) does not show UV emission but is
detected in three optical filters and in Hα; the third one
(lower left panel) is detected only in F680N, F275W and
Hα; all these three are therefore confirmed star-forming
clumps. The last one (lower right panel) shows emission
only in F680N and Hα and was therefore rejected.
Outside the stellar disk, Astrodendro is more prone
to detect residual cosmic rays and noise peaks as clump
candidates in the tails. Cosmic rays and noise peaks
are typically compact and bright, like the star-forming
clumps we aim to study. For these reasons, we perform
an additional check for clumps in the tails. Both the Hα-
and UV-selected tail clumps are matched with the cor-
responding catalogs of Hα clumps detected with MUSE
observations and described in Poggianti et al. (2019). If
a match is found, the HST clump is validated; if not,
the clump is validated only if either the redshift from
6 The 2σ threshold varies from galaxy to galaxy, ranging from 1.30
to 1.62 × 10−20 erg/s/cm2/Å for the F275W, and from 1.78 to
2.20 × 10−18 erg/s/cm2 for the Hα. More details on how these
values are computed can be found in Gullieuszik et al. (2023)
7 Defined as the ratio between the total flux of the clump and the
noise of the image in an area as large as that of the clump.
8 The reason for this is that a star-forming clump might be in
principle bright in UV and Hα only.
8. 8 GASP team
NO
YES
Clumps are detected
with Astrodendro
Is the
clump out of the
galactic disk and not
co-spatial with one of Hα clumps
detected with MUSE
(Poggianti et al.
2019)?
Is it possible
to infer the redshift of
the clump from MUSE
spectra?
Is it
consistent with that
of the host
galaxy?
Is SNR>2
in ALL the
filters?
YES
YES
YES
Is SNR>2 in
more than 2 photometric
filters?
NO
NO
YES
NO
Rejected
Confirmed
NO
-3611 Hα
-2293 UV
-4 Hα
-30 UV
-69 Hα
-186 UV
6090 Hα
6259 UV
Figure 4. Flow chart summarizing the selection procedure adopted in this paper to confirm (green checkmark) or reject (red
cross) clump candidates detected by Astrodendro.
the MUSE spectrum at the corresponding position of
the clump is consistent with that of the galaxy, or no
redshift can be inferred from the MUSE spectrum, but
the clump is detected in all HST filters. To infer the red-
shift, emission lines such as the [NII] 6548, 6583−Hα and
Hβ − [OIII] 4958, 5006 triplets and the [SII] 6716, 6730
doublet are fitted to the MUSE spectra, obtained within
a circular aperture as close as possible to that of the
clump. 73 Hα and 216 UV candidates were rejected
after this selection.
Finally, 5 UV-selected trunk clumps in the disks of the
JO201, JO204, JO206 and JW100 are removed as their
sizes and morphologies are such that they cannot be
considered clumps, rather than more likely entire parts
of the stellar disks.
For studying sizes, we define a sub-sample (resolved
sample) of resolved clumps9
by selecting those objects
9 When possible, we substitute unresolved leaf clumps with its
smallest, resolved, parent branch clump, if it does not contain
another resolved leaf clump.
9. AASTeX v6.3.1 Sample article 9
Figure 5. Images of 4 Hα clumps of JO206 in all the filters. Each clump is shown in 6 different filters, which are (from upper
left to lower right): F275W, F336W, F606W, F680N, F814W, Hα. Each filter is labelled in green or red whether we have a
detection or not, according to our definition (Sec. 4.2). The FOV is constant for all the clumps (a lengthscale equal to 250 pc
is plotted in lower left panel of the third clump).
whose PSF-corrected core radius, rcore,corr, exceeds the
PSF FWHM (0.0700
), which corresponds to ∼ 140 pc at
the typical redshifts of our targets.
Furthermore, when specified in the following, we re-
moved regions whose emission is powered by an AGN. In
order to do that, we used the BPT maps (Baldwin et al.
1981) of the MUSE images of the corresponding galaxies
(Poggianti et al. 2017b). Adopting the boundary lines
by Kauffmann et al. (2003), Kewley et al. (2001) and
Sharp & Bland-Hawthorn (2010), the MUSE spaxels
were flagged as star forming, composite, AGN or LINER
regions according to the line ratios log([NII]/Hα) and
log([OIII]/Hβ) (for the spaxels with S/N> 3 for each
line). The HST clumps are flagged as the MUSE spax-
els they fall into and in the following we remove those
flagged as AGN or LINER when studying the luminosi-
ties of the Hα-selected clumps.
4.3. Star-forming complexes
UV- and Hα-selected clumps probe the emission com-
ing from or due to stars younger than ∼ 108
yr and
∼ 107
yr, respectively. The contribution from stellar
components older than such timescales can be detected
in other optical bands used in this analysis, in order
to trace the whole stellar populations formed from the
stripped gas in the tails.
Therefore we decided to run Astrodendro also on
the F606W filter images, which are deeper than the UV
images (see Gullieuszik et al. 2023) and are sensitive
to older stellar populations with respect to F275W and
Hα. Details of the Astrodendro run on the F606W
images are given in Appendix A. Only tail trunk clumps
are considered, and we retain only F606W clumps over-
lapping with at least one pixel of any star-forming clump
in either the Hα-selected or the UV-selected samples. In
the following, we call star-forming complex the union of
a F606W clump and each star-forming clump matched
to it.
5. NUMBER OF CLUMPS: DISK, EXTRAPLANAR
AND TAIL CLUMPS
All the clumps are shown in Fig. 6 (the complete
figure set is available in the online journal). Here the
disk, extraplanar and tail clumps can be seen in differ-
ent colors, and their hierarchical, tree structure can be
appreciated from the color shading. In Fig. 7, we show
10. 10 GASP team
zoomed-in examples of Hα-selected clumps in JO201,
to illustrate the hierarchical structure and the irregular
morphologies of these clumps.
Fig. Set 6. Hα- and UV-selected clumps de-
tected in our sample of galaxies.
The largest clumps are found in the disks of JO175,
JO201 and JO206 and in the extraplanar region of
JO206. As shown by Fig. 7, large disk clumps (red
contours) typically contain several sub-clumps (yellow
contours), while extraplanar and especially tail clumps
often have only one level. One can also appreciate the ef-
fects of RPS on extraplanar clumps, like the filamentary
structures in JO206 and JW100, which are particularly
bright in UV (lower right panels in Figs. 6.4 and 6.6).
In the tails, clumps are often aligned in extended
linear or arched structures, suggesting the presence of
many sub-tails in each galaxy (as already noticed in
Bellhouse et al. 2021 and Franchetto et al. 2020, who
found sub-tails in these galaxies from MUSE images).
Whether clump and complex properties correlate with
distance from the galaxy or along its sub-tails and how
they are influenced by the properties of the hosting
galaxy are beyond the scope of this paper and will be
investigated in future works.
Fig. 8 shows a zoomed-in F606W image of some
structures in the tails of JO201, to better appreciate
the different spatial distributions of star-forming com-
plexes (dark violet contours), Hα-selected clumps (dark
orange) and UV-selected clumps (violet). Typically,
an Hα-selected clump has a corresponding UV-selected
clump, while the viceversa is not true. Indeed, the num-
ber of UV-selected clumps is higher than the number
of Hα-selected ones (see below). Furthermore, the cor-
responding UV-selected clump is generally bigger and
almost completely encompasses the Hα-selected clump.
Similarly, star-forming complexes contain many UV-
and Hα-selected clumps, embedded in fainter, optical
regions.
The number of star-forming clumps and complexes per
galaxy is given in Table 3 and Table 4, respectively. In
total, including all galaxies, our LT sample comprises
2406 Hα-selected clumps (1708 disk clumps, 375 extra-
planar clumps and 323 tail clumps), 3745 UV-selected
clumps (2021 disk clumps, 825 extraplanar clumps and
899 tail clumps) and 424 star-forming complexes. Typi-
cally, 98-99% of the selected clumps are leaves (including
also simple trunks with no substructures inside), while
the trunks containing leaves represent only 1-2% of the
whole sample (the fraction increases to 7-14% when re-
stricting the analysis only to resolved clumps).
Avoiding AGN areas and including both resolved and
unresolved clumps, ∼ 21% of the Hα-selected and ∼ 7%
of the UV-selected clumps get excluded. The percent-
age is smaller in the latter, indicating that UV-selected
clumps are more preferentially located out of AGN re-
gions than Hα-selected clumps. Most of these are disk
clumps, as expected, but a few of them can be found
in the ionization cone of the AGN, whose extension can
reach into the extraplanar region. The exact numbers
are listed in brackets in Table 3.
Only 12% of Hα-selected and 16% of UV-selected
clumps are spatially resolved, which means that the ma-
jority of the clumps have diameters smaller than ∼ 140
pc. Most of the resolved clumps are star-forming accord-
ing to the BPT, except in the disk where about 25% of
the Hα-resolved clumps are flagged as AGN or LINER.
In Fig. 9 we plot the histograms of the number of
clumps per galaxy, divided according to the selection
band (Hα or UV) and the spatial category (disk, extra-
planar and tail), together with the number of complexes.
In most cases disk clumps are much more numerous
than extraplanar and tail clumps, regardless of the selec-
tion filter, with the only exception being the UV-selected
clumps in JW100, which is seen edge-on and is stripped
mostly on the plane of the sky (Poggianti et al. 2019),
thus in the most favorable conditions to appreciate the
extraplanar clumps. For what concerns the number of
extraplanar and tail clumps, the prevalence of one over
the other depends on the galaxy: in JW100 the number
of extraplanar clumps is much larger than that of tail
clumps in both the selection filters, in JO204 and JO206
they are almost of the same number, while in the other
galaxies tail clumps are more numerous than extrapla-
nar. The number of clumps in each category clearly
depends on both the disk inclination and the stripping
direction with respect to the line of sight.
Furthermore, with the only exception being disk
clumps in JO175, for the same spatial category there are
more UV-selected clumps than Hα-selected ones. This
indicates that there are a number of stellar-only clumps,
with little or no ionized gas left.
The number of star-forming complexes in the tails of
the galaxies is generally smaller than the number of tail
UV-selected clumps, but larger than that of tail Hα-
selected ones, with the only exception being JO206, sug-
gesting that many complexes are matched only to UV-
selected clumps, without any Hα counterpart.
6. DISTRIBUTION FUNCTIONS
The luminosity (size) distribution function (LDF and
SDF hereafter) is defined as the number of sources per
luminosity (size) bin normalized by the width of the lu-
11. AASTeX v6.3.1 Sample article 11
Figure 6. Map of the clumps detected in JO201, superimposed onto the image in the filter used for the detection. Upper
panels: Hα-selected clumps. Lower panels: UV-selected clumps. Left panels: field of view including all the clumps. Right
panel: zoomed-in version on the vicinity of the disk (highlighted in the left panel with the black dashed rectangle). Colors
in the right panels represent the spatial category and the tree structure (Sec. 3 and 4.1): disk clumps are plotted in red
(trunks which are not leaves), orange (trunks which are leaves) and yellow (leaves which are not trunks). Similarly, extraplanar
clumps are plotted in dark blue, blue and light blue, and tail clumps are plotted in dark green, green and light green in
right panels. The grey dashed contour is the galaxy disk contour (see Sect. 3. In the left panels, for the sake of clarity,
the tail clumps are plotted as green dots of fixed size. The regions highlighted and labelled as A, B, C and D are shown in figure 7.
The complete figure set (6 images) including also the other galaxies is available in the online journal.
12. 12 GASP team
Table 3. Number of clumps in each sub-sample.
LT sample resolved sample
Filter gal NLT Nd Ne Nt nres nd ne nt
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
JO175 290(290) 252(252) 14(14) 24(24) 38(38) 37(37) 0(0) 1(1)
JO201 663(476) 507(321) 51(49) 105(105) 115(88) 96(69) 4(4) 15(15)
JO204 373(296) 290(219) 44(38) 39(39) 40(30) 32(23) 5(4) 3(3)
Hα JO206 438(377) 234(173) 117(117) 87(87) 44(37) 24(17) 19(19) 1(1)
JW39 235(168) 192(125) 4(4) 39(39) 14(12) 11(9) 0(0) 3(3)
JW100 407(284) 233(139) 145(116) 29(29) 35(18) 24(9) 9(7) 2(2)
tot 2406(1891) 1708(1229) 375(339) 323(323) 286(223) 224(164) 37(34) 25(25)
JO175 287(287) 211(211) 17(17) 59(59) 47(47) 38(38) 2(2) 7(7)
JO201 1244(1100) 659(518) 213(209) 372(372) 233(211) 143(122) 29(28) 61(61)
JO204 531(475) 302(258) 110(102) 119(115) 82(78) 63(59) 9(9) 10(10)
UV JO206 741(733) 392(384) 186(186) 163(163) 106(104) 51(49) 41(41) 14(14)
JW39 355(349) 243(237) 6(6) 106(106) 31(31) 26(26) 0(0) 5(5)
JW100 587(533) 214(178) 293(274) 80(80) 92(78) 48(36) 37(35) 7(7)
tot 3745(3476) 2021(1787) 825(794) 899(895) 591(549) 369(330) 118(115) 104(104)
Note—Number of clumps detected in each galaxy and depending on the spatial category. From left to right:
photometric band in which the clumps were detected (column 1), name of the galaxy (2), number of LT clumps
(3), number of disk-LT clumps (4), number of extraplanar-LT clumps (5), number of tail-LT clumps (6), number
of resolved clumps (7), number of resolved-disk clumps (8), number of resolved-extraplanar clumps (9), number
of resolved-tail clumps (10). In brackets, the number of clumps in the each sample, but selected in order to avoid
regions powered by AGN emission (see Sect. 4.2).
13. AASTeX v6.3.1 Sample article 13
Figure 7. Hα images of the JO201 regions highlighted in
the upper right panel of figure 6. The colors are the same of
figure 6.
Figure 8. Zoomed-in F606W image of some star-forming
complexes and clumps in JO201. Hα-selected clumps are
plotted in dark orange, UV-selected clumps in violet and
complexes in dark violet.
Table 4. Number of star-forming complexes detected in the
tails of each galaxy.
gal N
JO175 31
JO201 129
JO204 53
JO206 92
JW39 73
JW100 46
tot 424
minosity (size) bin itself and by the total number of
sources in the sample and is a useful tool to study the
statistical properties of the star-forming clumps. As de-
scribed is Sec. 1, they are typically well described by a
power law, and seem to be good proxies of the environ-
mental effects on the star-formation process and on the
clustering properties of the clumps.
6.1. Luminosity distribution functions
Figs. 10 and 11 show the histograms of the clumps
in each spatial category and in each galaxy, binned in
luminosity. The y-axis of plots are normalized by the
total number of clumps in the spatial category and in
the galaxy. Most Hα-selected clump distributions are
peaked at values fainter than ∼ 1038
erg/s, at the faint-
end of the luminosity dynamical range. The luminosi-
ties of the Hα-selected clumps are consistent with those
of ”giant” HII regions (like the Carina Nebula), whose
Hα luminosities L(Hα) are typically 1037−39
erg/s, and
”super giant” HII regions (like 30 Doradus in the Large
Magellanic Cloud), with L(Hα) > 1039
erg/s (Lee et al.
2011). As expected, the faintest clumps are observed
mostly in the closest galaxies of our sample (Table
1). JO201 stands out for its population of bright Hα-
selected clumps, both in the disk and in the tail, while in
the extraplanar regions the brightest clumps are those of
JO206 (located in the crest to the top right of the disk,
see Fig. 6.4). Similar trends are found for UV-selected
clumps. Also, we point out the hint for a bimodality
in the distributions of the disk UV-selected clumps of
JO201 and JW100 and of the extraplanar clumps of
JO206. Finally, the star-forming complexes distribu-
tions are very different from the others, since they do
not peak in the faint-end of the distribution.
As done in Cook et al. (2016), throughout this work
the datapoints of the luminosity distribution functions
de
N/dL (LDF) are computed fixing the number of ob-
jects while varying the bin size, in order to obtain a
robust representation of the distribution function. For
our LDFs we choose 20 sources per bin. The luminosity
14. 14 GASP team
Figure 9. Histograms of the number of star-forming clumps and complexes in each galaxy, divided according to the selection
filter and the spatial category. For each galaxy three panels are shown, with the number of Hα-selected clumps (left panel),
UV-selected clumps (middle panel) and star-forming complexes (right panel), divided according to their spatial category: disk
(red), extraplanar (blue) and tail (green).
of each bin is the central luminosity of the bin. Data-
points brighter than a given peak luminosity Lpeak are
fitted10
by a power law
p(L) = K L−α
with L ≥ Lpeak, (2)
where K is the normalization and α is the slope of the
power law. Lpeak is chosen for each sub-sample starting
from the peak value of the LDF and, if necessary, varying
it in order to avoid noisy regions of the LDF.
Hα-selected clumps, UV-selected clumps and star-
forming complexes are fitted independently in each spa-
tial category, in order to study variations in the proper-
ties of the LDFs as a consequence of RPS. We used the
whole UV-selected and star-forming complexes samples,
but only the BPT-selected Hα-selected clumps, in order
to avoid AGN- and LINER-powered regions (see Sec.
4.2). Fits were performed using the curve fit method
10 Varying the number of clumps per bin between 5 and 50, the
slope of the fit to the LDF does not change significantly. Using
a different fitting method which does not depend on the binning
(PowerLaw, Alstott et al. 2014; Clauset et al. 2009; Klaus et al.
2011), results do not change significantly either.
implemented in the SciPy11
Python package, with un-
certainties on the LDF computed as the Poisson noise
of the number of objects in the bin.
In Figs. 12 and 13 we plot the observed LDFs to-
gether with the corresponding best-fitting power laws.
Tail LDFs seem to be well described by a single power
law, both for Hα- and UV-selected clumps.
In Table 5 we list the best-fitting values of the slopes
α and the normalizations K, together with the peak lu-
minosities Lpeak. Considering all the cases, the value of
the slope α is in the range from 1.61 to 1.88 (thus al-
ways smaller than 2), with a mean value of 1.79 ± 0.09
(1.84 ± 0.03 for Hα-selected clumps and 1.73 ± 0.09 for
UV-selected clumps). In order to rule out the possi-
bility that the inclusion in the sample of trunk clumps
with sub-clumps can bias the results, we performed the
same fits to the LDFs excluding them. Since this kind
of trunks is ∼ 2% of the whole sample, excluding them
does not affect significantly the results and the leaf-only
11 https://docs.scipy.org/doc/scipy/index.html
15. AASTeX v6.3.1 Sample article 15
Figure 10. Fraction of Hα-selected (top row) and UV-selected (bottom row) clumps per spatial category and per galaxy. In
each row, from left to right: disk, extraplanar and tail. Y-axis is normalized for the number of clumps in the galaxy and in the
spatial category. Notice that the Hα luminosity of the Hα-selected clumps is the integrated emission of the Hα line, therefore
in erg/s, while the UV luminosity of the UV-selected clumps is in erg/s/Å.
slopes are always consistent within 1σ with those ob-
tained including both trunks and leaves.
Our LDF slopes are consistent with previous results
for HII regions (Kennicutt et al. 1989: 2.0 ± 0.5) and
are smaller than 2. Similar results have been found by
Santoro et al. (2022) for HII regions and Cook et al.
(2016) for UV-selected young stellar clusters.
We also performed a KS-test on the luminosity distri-
butions for pairs of spatial categories, for Hα- and UV-
selected clumps separately, to infer whether the LDF
significantly changes from one region to another. We
compare the distributions above the maximum Lpeak
value above which we can assume all three sub-samples
to be complete. The resulting P values are listed in the
Appendix (Table A2) and are consistent with what one
would expect when comparing the slopes of the LDFs.
For Hα-selected clumps, where the slopes are consistent
with each other within the errors, the KS-test cannot
exclude that each pair of distributions are identical. For
UV-selected clumps, the KS-test confirms significant dif-
ferences for the pairs disk-extraplanar and disk-tail.
In the top panel of Fig. 14 we show the comparison
among the best-fitting slopes, as a function of the selec-
tion band and the spatial category. Both the UV and
Hα slopes steepen going from disk, to extraplanar to
tail regions, where the closest match with the expected
slope α = 2 is found.
Shallower LDFs are found in galaxies with high sSFR
(Santoro et al. 2022), such as all our jellyfish galaxies
(Vulcani et al. 2018), which may explain why our slopes
are smaller than 2. Furthermore, as described in Sec.
1, past works (Cook et al. 2016; Messa et al. 2018; San-
toro et al. 2022) find flatter LDFs in environments with
a high SFR surface density ΣSFR. Whether tails and
disks are characterized by different ΣSFR is a matter of
future works, where masses and SFR od the clumps will
16. 16 GASP team
Table 5. Luminosity and size distribution functions best-fitting parameters.
α KL Lpeak αs Ks sizepeak
[erg/s(/Å)] pc
(1) (2) (3) (4) (5) (6) (7) (8)
D 1.82 ± 0.05 30.9 ± 1.8 1 × 1038
2.8 ± 0.2 4.2 ± 0.4 150
Hα E 1.83 ± 0.06 31 ± 2 7 × 1037
3.6 ± 0.6 6.4 ± 1.4 200
T 1.88 ± 0.04 33.0 ± 1.6 1 × 1038
4.4 ± 0.8 8 ± 2 160
D 1.61 ± 0.03 21.8 ± 1.2 8 × 1035
2.95 ± 0.16 4.6 ± 0.4 190
UV E 1.76 ± 0.04 27.3 ± 1.6 8 × 1035
2.9 ± 0.3 4.4 ± 0.7 200
T 1.83 ± 0.03 26.9 ± 1.1 8 × 1035
3.5 ± 0.4 5.9 ± 0.9 175
Complexes 1.83 ± 0.08 30 ± 3 1.7 × 1036
- - -
Note—List of the best-fitting values of the LDFs and SDFs when fitted to the different
samples of star-forming clumps and complexes. Column (1) refers to the clump selection
photometric band. Column (2), from top to bottom: disk (D), extraplanar (E) and tail
(T) sub-samples of Hα-selected and UV-selected clumps; the last row refers to star-
forming complexes, which are only in the tails by construction. Columns (3), (4) and
(5) contain the values of the best-fitting slopes α, the best-fitting normalization KL
and the peak luminosity Lpeak arbitrarily chosen, over which datapoints are fitted (Eq.
2). Notice that Lpeak is in erg/s for Hα-selected clumps, whereas it is in erg/s/Å for
UV-selected clumps and star-forming complexes. Columns (6), (7) and (8) list the same
quantities (best-fitting slope αs, best-fitting normalization Ks and peak size sizepeak),
but for the SDFs.
Figure 11. Same as Fig. 10, but for star-forming complexes.
be found by SED fitting. Projection effects and blend-
ing, which are more likely to affect the disk than the
tails, have also been demonstrated to flatten the LDF (as
demonstrated by Dessauges-Zavadsky & Adamo 2018 in
the case of mass distribution function). Flatter slopes
are also found in simulations that include the ageing ef-
fect of the most massive clumps (Gieles 2009; Fujii &
Portegies Zwart 2015), which would be consistent with
the fact that the slopes of the Hα-selected clumps (cir-
cles in Fig. 14) are larger than those of the UV-selected
clumps (squares) of the corresponding spatial category.
It would be also confirmed by the slope of the star-
forming complexes (all of which are located in the tails
by construction), which is very close to that of tail UV-
selected clumps.
Our analysis therefore suggests that the tails contain
proportionally fainter clumps than the disks, and the
extraplanar regions are intermediate between the two.
However, this difference is statistically significant only
when comparing UV-selected disk clumps with the other
spatial categories, while for Hα-selected clumps there
are only hints of such trend (Fig. 14). Furthermore,
observational biases could explain the shallower LDF
observed in disk clumps, since disk clumps are expected
to be more affected by blending effects and underlying
disk contamination, while the tail clumps are the least
contaminated population, being isolated. Hence their
observed LDF should be the closest to the intrinsic one.
Indeed, it is the closest to the theoretical expected value
of 2 (Elmegreen 2006). Thus, we can conclude that
the properties of the gas in which clumps are embed-
17. AASTeX v6.3.1 Sample article 17
Figure 12. Luminosity distribution functions de
N/dL of Hα-selected (upper panels) and UV-selected (lower panels) clumps.
Clumps are divided according to their spatial category: disk (left panels, in red), extraplanar (middle panels, in blue) and tail
(right panels, in green). For each plot we show: the empirical LDF of the corresponding sample (open circles with errorbars),
generated with equal-number bins (i.e. each bin contains the same number of objects, see Cook et al. 2016), and the best-fitting
line (dashed line). Notice that the Hα luminosity of the Hα-selected clumps is the integrated emission of the Hα line, therefore
in erg/s, while the UV luminosity of the UV-selected clumps is in erg/s/Å.
Figure 13. Same as Fig. 12, but for star-forming complexes.
ded are likely to play a minor role in influencing the
LDF. Nonetheless, this analysis cannot fully exclude ef-
fects on other properties of the clumps, like the mass,
which we will investigate in future works.
6.1.1. Deviation from single power law
Carefully inspecting Fig. 12, it is evident that disk
(and to some extent, also extraplanar) LDFs show some
particular features, such as slope changes, plateaus and
secondary peaks, hinting to the need of a more com-
plex model rather than a single power law. To char-
acterize these different regimes, disk LDFs are divided
in three intervals: the faint-end interval, the plateau
and the bright-end interval, each fitted with a power-
law. Furthermore, for the Hα-selected LDF we fitted a
power law also to datapoints brighter than 1.2 × 1039
erg/s, in correspondence of a secondary peak of the
18. 18 GASP team
Figure 14. Comparison of the slopes of the LDFs (left
panel) and SDFs (right panel) of star-forming clumps and
star-forming complexes, both as a function of the selection
band and the spatial category (see Table 5). Circles are Hα-
selected clumps, squares are UV-selected clumps and trian-
gles are star-forming complexes. Colors refers to the spatial
category: red for disk, blue for extraplanar and green for
tail.
LDF12
(”secondary-peak interval”, hereafter). We su-
perimposed the best-fitting disk power laws to the extra-
planar LDFs, in order to understand if also this spatial
category could be characterized by the same regimes (we
do not have enough statistics to divide the extraplanar
LDFs in intervals and fit a power law in each of them).
12 This secondary peak is dominated by clumps in JO201 (the
galaxy with the largest amount of disk and tail clumps).
Nonetheless, we do not have reasons to think there is a bias in
luminosity artificially increasing the number of clumps at such
luminosity, therefore it is a matter of interest to characterize this
interval, too.
The best-fitting slopes and the luminosity range
boundaries of each interval are shown in Table 6 and
in Fig. 15 we show the best-fitting power laws superim-
posed onto the disk and extraplanar LDFs.
For what concerns Hα-selected LDFs, the faint-end in-
terval slope is larger than that of the bright-end interval,
hinting to a change in the properties of the clumps before
and after the plateau. When considering the secondary-
peak interval, the distribution gets steeper than for the
bright-end interval, but still flatter than at the faint-
end. When superimposing these results on the extra-
planar LDF (right-end panels in Fig. 15), we can notice
that the faint- and bright-end best-fitting power laws
describe quite well the distribution. On the other hand,
the extraplanar LDF seems to lack the plateau and the
secondary-peak, even though we do not have enough
datapoints in these intervals to exclude this hypothesis.
Concerning the disk UV-selected LDF, the slopes in
the faint- and bright-end intervals are consistent within
the uncertainties. The plateau covers a wider luminosity
range compared to the Hα plateau. The presence of a
plateau in UV LDFs has never been observed before.
Furthermore, the extraplanar LDF is well described by
the results obtained for the disk, especially in the faint-
end interval.
Whether these different regimes are an effect of the
ageing or not is not clear yet. The position of the
plateau in the disk Hα LDF is compatible with a change
in the bounding regime (from density bound to ioniza-
tion bound) of the HII regions (Beckman et al. 2000) at
a predicted Hα luminosity (at ∼ 4×1038
erg/s). On the
other hand the slopes at the low and high luminosity
ends are similar, while the Beckman et al. (2000) model
predicts a steepening at bright luminosities, where HII
regions are ionization bound. Moreover, our LDFs show
the same plateau also in the disk UV-selected clumps,
which should not be affected by the changing in the ion-
ization regime.
6.2. Size distribution functions
In this section we use the clumps of the resolved sam-
ple(s). The analysis of the size distribution functions of
the clumps (SDFs hereafter) is performed in the same
way described in Sect. 6.1. The samples are binned us-
ing 15 sources per bin for disk clumps and 5 sources per
bin for the extraplanar and tail clumps, because of the
low number of clumps in these spatial categories. SDFs
are qualitatively similar to LDFs. Their intrinsic func-
tional form is a power law, but incompleteness effects
introduce a cutoff at small sizes. In analogy with what
19. AASTeX v6.3.1 Sample article 19
Table 6. Best-fitting slopes to the intervals of the disk LDFs.
Phot. band Parameters Faint-end Plateau Bright-end Secondary-peak
α 2.7 ± 0.2 1.1 ± 0.4 1.82 ± 0.08 2.04 ± 0.08
Hα Lmin [erg/s] 1 × 1038
2.7 × 1038
5 × 1038
1 × 1039
Lmax [erg/s] 2.7 × 1038
5 × 1038
- -
α 2.24 ± 0.06 0.98 ± 0.14 2.31 ± 0.13 -
UV Lmin [erg/s/Å] 8 × 1035
6 × 1036
3 × 1037
-
Lmax [erg/s/Å] 6 × 1036
3 × 1037
- -
Note—Results obtained when a set of power laws are fitted to the disk LDFs divided
in intervals (Sect. 6.1). From left to right: clump selection photometric band (Phot.
band); best-fitting slope α and luminosity range boundaries of the interval Lmin and Lmax
(Parameters); names of the intervals (Faint-end, Plateau, Bright-end and Secondary-peak).
Figure 15. Luminosity distribution functions de
N/dL of Hα-selected (upper panels) and UV-selected (lower panels) clumps.
Clumps are divided according to their spatial category: disk (left panels, in red), extraplanar (middle panels, in blue) and tail
(right panels, in green). For each plot we show: the empirical LDF of the corresponding sample (open circles with errorbars),
generated with equal-number bins (i.e. each bin contains the same number of objects, see Cook et al. 2016), and the best-fitting
line (dashed line). Notice that the Hα luminosity of the Hα-selected clumps is the integrated emission of the Hα line, therefore
in erg/s, while the UV luminosity of the UV-selected clumps is in erg/s/Å.
20. 20 GASP team
we did for the LDF (Eq. 2), we define the peak value as
sizepeak and we fit a power law to datapoints above this
value.
In Fig. 16 the observed SDFs and the best-fitting
model of each sub-sample are shown. For completeness,
we plot also the SDF datapoints of unresolved clumps,
for which we have only upper limits for the sizes (filled
dots). In order to do that, SDFs are not normalized
for the total number of clumps, since the normalization
changes when considering unresolved clumps or not. A
single power law is likely to be a good representation of
the resolved data, especially considering that the sample
is about 15% of the one used to constrain the parameters
of the LDFs (see Table 3). The loss of statistics can
affect especially the extraplanar and tail sub-samples,
for which the regime in which the sample is complete
includes just a few datapoints. The fitted power laws do
not seem to well describe the unresolved datapoints, as
expected from incompleteness. These features, together
with the fact that unresolved clumps have, by definition,
no reliable estimates of their sizes, imply that we cannot
draw any conclusion for sizes below ∼ 140 pc.
The best-fitting slopes and normalizations, and the
chosen sizepeak of each sub-sample are listed in columns
(6), (7) and (8) of Table 5. The average slope is 3.3±0.6
(3.6 ± 0.6 for Hα-resolved clumps and 3.1 ± 0.3 for UV-
resolved clumps). Slopes of extraplanar and tail Hα-
resolved clumps are consistent with the one found by
Kennicutt & Hodge (1980) in the disk of a low-z spiral
galaxy (α = 4.1).
As done in Sect. 6.1 for LDFs, we computed the P
values from the KS statistics comparing the size distri-
butions of pairs of spatial categories, keeping the two se-
lection filters separated. Results are listed in Appendix
(Table A2). In this case, the KS finds significantly dif-
ferent distributions for all pairs, except for disk vs tail
Hα-resolved clumps. However, both the slopes and the
P values have to be taken with caution, due to small
numbers, especially in the tail clumps.
We find that these distributions are different from
those inferred for the Hα clumps of these galaxies de-
tected by (Poggianti et al. 2019) from the MUSE Hα
luminosities using the luminosity-size relation by Wis-
nioski et al. (2012): the expected median size was 440
pc for clumps in the disks and 320 pc for clumps in
the tails. Here, the median sizes are ∼ 210 pc, ∼ 211
pc, ∼ 180 pc for disk, extraplanar and tail Hα-resolved
clumps, respectively, and ∼ 215 pc, ∼ 223 pc, ∼ 208
pc for disk, extraplanar and tail UV-resolved clumps.
Consistently with what was inferred by Poggianti et al.
(2019), clumps in the tails are smaller than those in
the disk. Nonetheless, values found in this work are
about half the expected size. The origin of this differ-
ence is a direct consequence of the differences between
luminosity-size relation by Wisnioski et al. (2012) and
the one obtained from our HST observations (see Sect.
7).
In the bottom panel of Fig. 14 the slopes of resolved
clumps in each category are plotted. Also in the case of
SDFs the slope of UV-resolved clumps are smaller (with
the exception of disk clumps), even if consistent within
errorbars, than those of Hα-resolved clumps. More-
over, disk and extraplanar UV-resolved slopes are al-
most equal, while in Hα there are hints of a slope in-
crease from disk to tail regions.
The slope increase can be partially explained based
on the work in Gusev (2014), whose observations of the
nearby galaxy NGC 628 demonstrated that the overall
slope of SDFs reaches values between 4.5 and 613
when
analysing the smallest structures of the star-forming re-
gions (i.e. what we define as leaves in Sec. 4.1) or iso-
lated objects. Instead, the slope decreases up to 2.5
once all the substructures of complex star-forming re-
gions are taken into account. Our trend is analogous.
We find steep slopes (∼ 4.4 ± 0.8, consistent with 4.5)
in the Hα tails, whose clumps have typically no or few
substructures. On the other hand, the slope is smaller
in the case of disk clumps, which are more structured
than extraplanar and tail clumps. Therefore, including
both trunks and leaves in the samples has little effects
on the slope of tail clumps, while it may explain the
flatter distribution found for disk clumps. Indeed, we
observe steeper disk SDFs when using only the leaves
(3.28±0.28 in Hα and 3.22±0.23 in UV). Alternatively,
recent simulations of star forming regions in presence
of different ambient pressures (Nath et al. 2020) found
slopes similar to the one of the disk SDFs, while they
suggest the presence of a lower pressure environment in
the tail. The pressure producing the measured steep-
ening in the tail SDF would be one order of magnitude
lower than the typical ICM pressure of our galaxies (Bar-
tolini et al. 2022). Therefore the variation of the slope
of the SDF across different environments seems to be
different to that expected from environmental effects.
In conclusion, the largest clumps of our sample are
found in the disk and in the extraplanar regions of
our galaxies, whether we consider UV- or Hα-resolved
clumps, and (as hinted by the KS-test), clumps of differ-
ent spatial categories are likely to follow different SDFs
13 Gusev (2014) studied the slopes of the cumulative distribution
functions. Also, their slopes are defined as negative. Therefore
the slopes in this work (αs) and the slopes by Gusev (2014) (αG)
are connected by αs = 1 − αG.
21. AASTeX v6.3.1 Sample article 21
Figure 16. Size distribution functions for disk (red), extraplanar (blue) and tail (green) clumps. Top: Hα. Bottom: UV.
Resolved clumps are shown as empty circles with 1σ errorbars, while unresolved clumps are plotted as filled circles without
errorbars. In this case, SDFs are not normalized by the total number of clumps and the x-axis is in linear scale.
with different slopes. The sizes of the clumps seem to
be poorly affected by the environment in which they are
embedded, ICM in the tails and ISM in the disks, and
more linked to their clustering features.
7. LUMINOSITY-SIZE RELATIONS
In this section we study the luminosity-size rela-
tion, both for Hα- and UV-resolved clumps. Here Hα-
resolved clumps are BPT-selected to avoid AGN- and
LINER-powered regions. To calculate the linear regres-
sion fits with the inclusion of an intrinsic scatter, we
employed the Python software package linmix (Kelly
2007). linmix implements a Markov Chain Monte Carlo
(MCMC) algorithm to converge on the posterior and re-
turn a sample of sets of parameters drawn from the pos-
terior distribution. The linear relation fitted by linmix
is
LogL = m Log(size) + q + G(ε), (3)
where L is the luminosity of the clump in the filter in
which it is selected, size is the PSF-corrected core di-
ameter, m is the angular coefficient of the correlation,
q is the y-axis intercept and G(ε) is the intrinsic scat-
ter, computed from a Gaussian distribution centered in
m Log(size) + q with standard deviation ε.
In Fig. 17 we plot the datapoints in the
(LogL, Log(size)) plane and the best-fitting lines, both
for Hα- and UV-resolved clumps (left and right panel,
respectively). Clumps are divided according to their
spatial position.
Best-fitting parameters are listed in Table 7. The av-
erage slope is 2.3±0.4 for Hα-resolved and 1.97±0.17 for
UV-resolved clumps. The slopes for the disk and extra-
planar Hα-resolved clumps are consistent within 1σ and
close to 2, while the slope for the tail clumps is steeper.
In UV, the slopes of all spatial categories are consistent
with each other.
22. 22 GASP team
Figure 17. Luminosity-size relations for Hα-resolved clumps (on the left) and UV-resolved clumps (on the right). The clumps
are plotted according to their spatial category: disk (red circles), extraplanar (blue stars), tail (green triangles). The best-fitting
lines to the three categories are plotted as solid lines of the corresponding color. The shaded areas are the uncertainties on the
fits at 2σ. Note that Hα luminosity is in erg/s, while F275W in erg/s/Å.
Table 7. Luminosity-size relations best-fitting parameters.
Hα UV
Spat. cat. m q ε m q ε
(1) (2) (3) (4) (5) (6) (7)
Disk 2.04 ± 0.12 34.6 ± 0.3 (8.5 ± 1.0) × 10−2
1.81 ± 0.12 33.2 ± 0.3 (14.0 ± 1.0) × 10−2
Extraplanar 1.9 ± 0.4 35.0 ± 0.8 (10 ± 3) × 10−2
1.90 ± 0.14 33.0 ± 0.3 (9.2 ± 1.3) × 10−2
Tail 2.9 ± 0.5 32.7 ± 1.2 (10 ± 3) × 10−2
2.2 ± 0.2 32.3 ± 0.4 (7 ± 1) × 10−2
Note—Best-fitting parameters (m, q, ε) (with 1σ uncertainties) of the luminosity-size linear relations (Eq. 3)
for Hα-resolved clumps (columns 2, 3, 4) and UV-resolved clumps (columns 5, 6, 7). Each spatial category
(column 1) is fitted separately.
The Strömgren sphere model predicts the slope to be
3 (Beckman et al. 2000), hinting that disk and extra-
planar clumps are not well described by this model as
a consequence of additional effects to be taken into ac-
count, such as RPS, transition from ionization bound
to density bound, dust, metallicity and magnetic fields
(Wisnioski et al. 2012). On the other hand, Nath et al.
(2020) obtained a slope equal to 2 simulating the ex-
pansion of ionized bubbles in a Milky Way-like ISM en-
vironment (in this case, Hα-resolved tail clumps would
be the only ones deviating from the prediction of the
model). Cosens et al. (2018) proposed a model explain-
ing why in galactic disks clumps with a low star forma-
tion rate surface density (ΣSFR) seem to follow a steeper
relation (slope closer to 3) than clumps with high ΣSFR
(slope closer to 2). According to this model, if the ex-
pected radius of a Strömgren sphere is larger than the
scale-height of the disk (H), the ionized bubble can keep
growing only across the galactic plane, according to a
power law with slope closer to 2 rather than 3. The flat-
tening occurs only if the ionized region is brighter than
a critical value. The fact that our slopes are consistent
with 2 might therefore suggest that our clumps have
an enhancement in the Hα luminosity, maybe caused by
ram-pressure stripping. Such a model would explain also
why Hα-resolved tail clumps are likely to follow a steeper
relation than clumps in the disk or extraplanar region.
Tail clumps are embedded in the spherically symmetric
ICM, in place of the gaseous disk of the galaxy, therefore
they are not bound by H.
7.1. Comparison with previous results
In Fig. 18 we compare the position of our Hα-resolved
clumps in the log L − log(size) with those presented in
the literature. We show results from Fisher et al. (2017),
23. AASTeX v6.3.1 Sample article 23
who study clumps belonging to turbulent, extremely
Hα-bright DYNAMO galaxies, and those by Wisnioski
et al. (2012), who studied z ∼ 0, isolated, star-forming
galaxies (Arsenault & Roy 1988; Kennicutt et al. 2003;
Rozas et al. 2006; Gallagher & Hunter 1983; Monreal-
Ibero, A. et al. 2007). We also show the best-fitting
relations they present in their works.
The luminosities of the DYNAMO clumps in Fig.
18 were corrected re-adding extinction caused by dust,
since both our luminosities and those computed by
Wisnioski et al. (2012) are not dust-corrected. Dust-
extincted DYNAMO clumps are then fitted to a power
law with slope fixed at 2, as done in Fisher et al. (2017).
As described in detail in Fisher et al. (2017), the radii
of DYNAMO clumps were found fitting a 2D Gaussian
to the light distribution, with the addition of a con-
stant representing the local background level (Fisher
et al. 2017). To make the comparison with DYNAMO
as consistent as possible, we derive new PSF-corrected
core radii (rgauss) fitting a 2D Gaussian+constant to
our tail Hα-resolved clumps, which are more isolated
and in a fainter local background than disk and extra-
planar clumps. We then visually select only clumps
for which a fit is appropriate. For these clumps, we
compute rgauss − rcore,corr, finding that it does not cor-
relate with rcore,corr, it ranges between 0 and 50 pc
and it has a median value of 25.5 pc. Assuming that
this difference is a good representation of the value of
rgauss − rcore,corr for all the Hα-resolved clumps of our
sample, we computed a new PSF-corrected core radius
e
rcore,corr = rcore,corr + 25.5 pc. Therefore the new sizes
are 2e
rcore,corr. The luminosities are re-computed inte-
grating the light within a circle of radius 3e
rcore,corr. The
procedure adopted in Wisnioski et al. (2012) to com-
pute luminosity and size is similar to the one applied
in Fisher et al. (2017), though not identical. Therefore
we are confident that the corrections we applied to our
clumps allow us to make a fair comparison also with the
results by Wisnioski et al. (2012).
Our clumps lie between the Fisher and Wisnioski re-
lations, being closer to the one obtained by Fisher et al.
(2017), even though they have lower luminosities and
sizes compared to the peak of their distribution. With
respect to the Wisnioski clump distribution, our re-
solved clumps are on average larger and, at a given size,
brighter.
As shown by Johnson et al. (2017) and Cosens et al.
(2018), DYNAMO clumps have both higher SFR and
ΣSFR than clumps in isolated spiral galaxies, as a con-
sequence of the starbursty star formation of their host-
ing galaxies. Being closer to the DYNAMO sample in
the luminosity-size relation may suggest our Hα-resolved
clumps to have a high ΣSFR, too (hints of that has al-
ready been found in Vulcani et al. 2020, in which they
studied the resolved SFR-stellar mass relation for the
MUSE Hα clumps).
8. CATALOG
We release the catalogs of Hα- and UV-selected
clumps, separately, as online Table. Each clump is uni-
vocally determined by the name of the galaxy, a letter
(referred to the Astrodendro run in which it has been
detected, see Sec. A) and an ID number. We then list
the RA and DEC coordinates, the luminosity in the se-
lection filter (not corrected for dust, but corrected for
NII in the case of Hα-selected clumps), the morpholog-
ical quantities (area, major and minor sigma, position
angle, core radius and PSF-corrected core radius), the
photometric fluxes and their errors in each band, in-
cluding F680N continuum-subtracted (Hα + NII), a flag
for the clump properties in the tree structure (0 trunks
which are not leaves, 1 trunks which are also leaves, 2
branches, 4 leaves which are not trunks), a flag for the
spatial category (0 tail, 1 extraplanar and 2 disk) and a
flag for the BPT classification (0 no BPT diagram avail-
able, 1 star-forming, 2 composite, 3 AGN, 4 LINER).
Details about how these quantities are computed are
given in Secs. 4.1 and 4.2.
As an example, in Appendix (Table A3) we report the
first ten rows of the Hα-selected clumps catalog. For
clarity, for some values not all the significant digits are
reported.
9. SUMMARY
In this paper we have built a sample of star-forming
clumps and complexes in six jellyfish galaxies, using a set
of HST images in five photometric bands. Clumps were
detected independently in UV (∼ 2700 Å) and Hα, in or-
der to probe star formation on different timescales (Ken-
nicutt 1998), while the star-forming complexes were de-
tected from optical emission (∼ 6000 Å) to fully recover
the stellar content formed from the stripped material in
each clump region. Clumps were also divided in three
spatial categories to study separately clumps formed
within the disk (disk clumps), clumps likely originated
in extraplanar gas but still close to the disk (extraplanar
clumps) and clumps formed in the stripped gas out of the
galactic disk and embedded in the ICM (tail clumps).
Also, clumps in the tail give the unprecedented opportu-
nity to study young stellar populations with no influence
or underlying contamination by the stellar disk.
The method we use to detect clumps (Sec. 4.1) yields
the hierarchical cascade structure of these star-forming
regions. While disk clumps are often characterized by
24. 24 GASP team
Figure 18. log(L(Hα)) − log(size) comparing our Hα-resolved clumps with those in DYNAMO starburst galaxies (Fisher et al.
2017, blue contours) and those in local, isolated, star-forming galaxies presented in Wisnioski et al. 2012 (Arsenault & Roy
1988; Kennicutt et al. 2003; Rozas et al. 2006; Gallagher & Hunter 1983; Monreal-Ibero, A. et al. 2007, black dots). Our clumps
are plotted according to their spatial category: disk (red circles), extraplanar (blue stars), tail (green triangles). Our clump
luminosities and sizes are corrected in order to make the comparison more trustworthy; for the same reason, DYNAMO clump
luminosities are corrected re-adding the effects of dust extinction. The black dashed line is the best-fitting relation by Wisnioski
et al. (2012), the blue dashed line is obtained fitting the dust-extincted DYNAMO clumps keeping the slope fixed at 2, as done
in Fisher et al. (2017). Clumps in our sample lie in between the two sample, being close in particular to clumps in starburst
galaxies.
complex structures where many clumps are localized in
bigger structures, extraplanar and tail clumps are typ-
ically simple structures with no sub-regions. Moreover,
tail clumps tend to be aligned in elongated or arched
structures along RPS sub-tails. Interestingly, the Hα-
selected clumps, UV-selected clumps and F606W star-
forming complexes are often nested into each other,
with Hα-selected clumps being embedded in larger UV-
selected clumps, and star-forming complexes containing
sometimes several UV- and Hα-selected clumps.
The final samples comprise 2406 Hα-selected clumps
(1708 disk clumps, 375 extraplanar clumps and 323 tail
clumps), 3745 UV-selected clumps (2021 disk clumps,
825 extraplanar clumps and 899 tail clumps) and 424
star-forming complexes (located only in the tails by
construction). The covered luminosity range goes from
∼ 3×1038
to ∼ 4×1040
erg/s in Hα, and from ∼ 3×1036
to ∼ 2 × 1039
erg/s/Å in UV. On average, ∼ 15% of
them are resolved, meaning that their sizes are larger
than ∼ 140 pc, up to more than 1 kpc.
We studied the luminosity distribution functions
(LDFs), size distribution functions (SDFs) and
luminosity-size relations of Hα- and UV-selected clumps
as a function of the spatial category. The LDF slopes av-
eraged on all the spatial categories are 1.84±0.03 for Hα-
selected clumps and 1.73±0.09 for UV-selected clumps.
The average slopes of SDFs are 3.6±0.6 for Hα-resolved
clumps and 3.1 ± 0.3 for UV-resolved clumps. Finally,
the average slopes of the luminosity-size relations are
2.3±0.4 for Hα-resolved clumps and 1.97±0.17 for UV-
resolved clumps. We find no clear difference among disk,
extraplanar and tail clumps. The best-fitting slopes of
these distributions and relations are consistent among
each other, as well as with the results obtained in previ-
ous works (Kennicutt et al. 1989; Cook et al. 2016; San-
toro et al. 2022) and with theoretical predictions of hier-
archical turbulence-driven star formation (Elmegreen &
25. AASTeX v6.3.1 Sample article 25
Falgarone 1996; Elmegreen 2006). On the other hand,
the luminosity-size relation of the Hα-resolved clumps
is more similar to that of clumps in starburst galaxies
and therefore it suggests that these clumps, regardless of
the spatial category, are experiencing an enhancement in
ΣSFR. These preliminary results suggest ram pressure
to compress the ISM and increase the Hα luminosity
of the clumps, while neither the presence of a disk and
its gravity, nor the gaseous conditions of the surround-
ing medium have a strong impact on the star formation
process, once the cold gas cloud conditions are set.
Future works on the mass, age and star formation of
the clumps, on trends and gradients with the distance
from the galaxies and on their fate will elucidate how
and how much these clumps differ from those in undis-
turbed galaxies, in order to shed light on the effects of
ram pressure on the galactic ISM and of environment on
star formation.
EG would like to thank A. Ignesti, A. Wolter, A.
Marasco, R. Smith, K. George and the GASP team for
the useful discussions and comments. We would like to
thank also Emily Wisnioski for providing us the data
from her paper. This paper is based on observations
made with the NASA/ESA Hubble Space Telescope ob-
tained from the Space Telescope Science Institute, which
is operated by the Association of Universities for Re-
search in Astronomy, Inc., under NASA contract NAS
5-26555. These observations are under the programme
GO-16223. All the HST data used in this paper can be
found in MAST: 10.17909/tms2-9250. This paper used
also observations collected at the European Organiza-
tion for Astronomical Research in the Southern Hemi-
sphere associated with the ESO programme 196.B-0578.
This research made use of Astropy, a community devel-
oped core Python package for Astronomy by the Astropy
Collaboration (2018). This project has received funding
from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation
programme (grant agreement No. 833824) and ”INAF
main-streams” funding programme (PI B. Vulcani).
APPENDIX
A. ASTRODENDRO PARAMETERS SETTING
Three parameters regulate how Astrodendro builds the tree structure:
• min value: the algorithm stops when the flux threshold reaches this value, instead of zero;
• min npix: minimum number of pixels for a clump to be included to the tree structure;
• min delta: the threshold is not lowered in a continuum way, but at steps of min delta. If no min delta is
given, the algorithm identifies each local maximum as a new sub-clump. min delta should be high enough to
avoid the detection of noise peaks in the surface brightness distribution as sub-clumps.
We performed three runs of Astrodendro for the F275W and Hα images of each galaxy, adopting the following
parameters (min npix= 5 in all the runs):
• RUN A: min value= 2.5σ; min delta= 5σ;
• RUN B: min value= 2.5σ; no min delta. Given that a clump candidate is detected only if its brightest pixel
is brighter than about min value+min delta, regions for which each pixel has counts between min value and
min value+min delta are not detected. Since we want to detect also these fainter clumps, we ran Astro-
dendro a second time without defining min delta. This run is executed on an image masked for the clumps
detected in run A and only trunk clumps are retained, to avoid including spurious local maxima.
• RUN C: min value= 2σ; no min delta. This run is performed on an image masked for the clumps detected
and kept in runs A and B. For the same reasons explained for run B, we kept only the trunks clumps of run
C. Also, as a consequence of removing the high-frequency components of the image, denoising introduces a
sort of smoothing, and part of the light of the brightest regions of the image, already detected as clumps, may
eventually smooth out of the masks defined from runs A and B. Thus, even masking the image for the clumps
26. 26 GASP team
Table A1. Summary of the number of clumps in each sub-sample used throughout
this paper, depending on the galaxy and on the spatial category.
LT sample resolved sample
Filter gal A B C A B C
(1) (2) (3) (4) (5) (6) (7) (8)
JO175 65(65) 214(214) 11(11) 31(31) 7(7) 0(0)
JO201 206(165) 438(296) 19(15) 97(81) 18(7) 0(0)
JO204 81(60) 287(233) 5(3) 36(29) 4(1) 0(0)
Hα JO206 99(84) 318(274) 21(19) 41(34) 3(3) 0(0)
JW39 27(20) 198(143) 10(5) 11(9) 3(3) 0(0)
JW100 94(40) 299(232) 14(12) 29(13) 6(5) 0(0)
tot 572(434) 1754(1392) 80(65) 245(197) 41(26) 0(0)
JO175 74(74) 204(204) 9(9) 35(35) 7(7) 0(0)
JO201 399(372) 779(670) 66(58) 198(180) 35(31) 0(0)
JO204 106(102) 397(348) 28(25) 68(65) 14(13) 0(0)
UV JO206 137(135) 573(568) 31(30) 82(80) 24(24) 0(0)
JW39 55(55) 282(276) 18(18) 28(8) 3(3) 0(0)
JW100 173(152) 392(362) 22(19) 80(67) 12(11) 0(0)
tot 949(891) 2627(2428) 174(159) 491(455) 100(94) 0(0)
Note—Table with the number of clumps detected in each galaxy and depending
on the Astrodendro run. From left to right: the photometric band in which
the clumps were detected (1), the name of the galaxy (2), the number of clumps
in the LT sample detected in runs A (3), B (4) and C (5), the number of clumps
in the resolved sample detected in runs A (6), B (7) and C (8). In brackets,
the number of clumps in the same sample, but selected in order to avoid regions
powered by AGN emission (see Sect. 4.2).
already detected, the residual smoothed emission adjacent to these masks may be possibly flagged as a clump in
run C. Since such an emission is clearly not due to a real clump, we excluded from the sample generated by run
C all the clump candidates adjacent to the clumps found in the previous runs.
In the case of F606W images, Astrodendro was run on the denoised F606W images, with min value= 3σ and
no min delta.
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