We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part
of the Survey of H I in Extremely Low-Mass Dwarfs (SHIELD) project, which is a multi-wavelength
study of galaxies with H I masses in the range of 106-107:2 M discovered by the ALFALFA survey.
We have obtained spectra of the lone H II region in AGC 198691 with the new high-throughput
KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m as well as with the Blue
Channel spectrograph on the MMT 6.5-m telescope. These observations enable the measurement of the
temperature-sensitive [O III]4363 line and hence the determination of a \direct" oxygen abundance
for AGC 198691. We nd this system to be an extremely metal-decient (XMD) system with an
oxygen abundance of 12+log(O/H) = 7.02 0.03, making AGC 198691 the lowest-abundance starforming
galaxy known in the local universe. Two of the ve lowest-abundance galaxies known have
been discovered by the ALFALFA blind H I survey; this high yield of XMD galaxies represents a
paradigm shift in the search for extremely metal-poor galaxies.
Detection of lyman_alpha_emission_from_a_triply_imaged_z_6_85_galaxy_behind_m...Sérgio Sacani
We report the detection of Ly emission at 9538A
in the Keck/DEIMOS and HST WFC3
G102 grism data from a triply-imaged galaxy at z = 6:846 0:001 behind galaxy cluster MACS
J2129.4 0741. Combining the emission line wavelength with broadband photometry, line ratio upper
limits, and lens modeling, we rule out the scenario that this emission line is [O II] at z = 1:57. After
accounting for magnication, we calculate the weighted average of the intrinsic Ly luminosity to be
1:31042 erg s 1 and Ly equivalent width to be 7415A. Its intrinsic UV absolute magnitude at
1600A
is 18:60:2 mag and stellar mass (1:50:3)107 M, making it one of the faintest (intrinsic
LUV 0:14 L
UV) galaxies with Ly detection at z 7 to date. Its stellar mass is in the typical range
for the galaxies thought to dominate the reionization photon budget at z & 7; the inferred Ly escape
fraction is high (& 10%), which could be common for sub-L z & 7 galaxies with Ly emission. This
galaxy oers a glimpse of the galaxy population that is thought to drive reionization, and it shows
that gravitational lensing is an important avenue to probe the sub-L galaxy population.
First identification of_direct_collapse_black_holes_candidates_in_the_early_u...Sérgio Sacani
The first black hole seeds, formed when the Universe was younger than ⇠ 500Myr, are recognized
to play an important role for the growth of early (z ⇠ 7) super-massive black holes.
While progresses have been made in understanding their formation and growth, their observational
signatures remain largely unexplored. As a result, no detection of such sources has been
confirmed so far. Supported by numerical simulations, we present a novel photometric method
to identify black hole seed candidates in deep multi-wavelength surveys.We predict that these
highly-obscured sources are characterized by a steep spectrum in the infrared (1.6−4.5μm),
i.e. by very red colors. The method selects the only 2 objects with a robust X-ray detection
found in the CANDELS/GOODS-S survey with a photometric redshift z & 6. Fitting their
infrared spectra only with a stellar component would require unrealistic star formation rates
(& 2000M# yr−1). To date, the selected objects represent the most promising black hole seed
candidates, possibly formed via the direct collapse black hole scenario, with predicted mass
> 105M#. While this result is based on the best photometric observations of high-z sources
available to date, additional progress is expected from spectroscopic and deeper X-ray data.
Upcoming observatories, like the JWST, will greatly expand the scope of this work.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part
of the Survey of H I in Extremely Low-Mass Dwarfs (SHIELD) project, which is a multi-wavelength
study of galaxies with H I masses in the range of 106-107:2 M discovered by the ALFALFA survey.
We have obtained spectra of the lone H II region in AGC 198691 with the new high-throughput
KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m as well as with the Blue
Channel spectrograph on the MMT 6.5-m telescope. These observations enable the measurement of the
temperature-sensitive [O III]4363 line and hence the determination of a \direct" oxygen abundance
for AGC 198691. We nd this system to be an extremely metal-decient (XMD) system with an
oxygen abundance of 12+log(O/H) = 7.02 0.03, making AGC 198691 the lowest-abundance starforming
galaxy known in the local universe. Two of the ve lowest-abundance galaxies known have
been discovered by the ALFALFA blind H I survey; this high yield of XMD galaxies represents a
paradigm shift in the search for extremely metal-poor galaxies.
Detection of lyman_alpha_emission_from_a_triply_imaged_z_6_85_galaxy_behind_m...Sérgio Sacani
We report the detection of Ly emission at 9538A
in the Keck/DEIMOS and HST WFC3
G102 grism data from a triply-imaged galaxy at z = 6:846 0:001 behind galaxy cluster MACS
J2129.4 0741. Combining the emission line wavelength with broadband photometry, line ratio upper
limits, and lens modeling, we rule out the scenario that this emission line is [O II] at z = 1:57. After
accounting for magnication, we calculate the weighted average of the intrinsic Ly luminosity to be
1:31042 erg s 1 and Ly equivalent width to be 7415A. Its intrinsic UV absolute magnitude at
1600A
is 18:60:2 mag and stellar mass (1:50:3)107 M, making it one of the faintest (intrinsic
LUV 0:14 L
UV) galaxies with Ly detection at z 7 to date. Its stellar mass is in the typical range
for the galaxies thought to dominate the reionization photon budget at z & 7; the inferred Ly escape
fraction is high (& 10%), which could be common for sub-L z & 7 galaxies with Ly emission. This
galaxy oers a glimpse of the galaxy population that is thought to drive reionization, and it shows
that gravitational lensing is an important avenue to probe the sub-L galaxy population.
First identification of_direct_collapse_black_holes_candidates_in_the_early_u...Sérgio Sacani
The first black hole seeds, formed when the Universe was younger than ⇠ 500Myr, are recognized
to play an important role for the growth of early (z ⇠ 7) super-massive black holes.
While progresses have been made in understanding their formation and growth, their observational
signatures remain largely unexplored. As a result, no detection of such sources has been
confirmed so far. Supported by numerical simulations, we present a novel photometric method
to identify black hole seed candidates in deep multi-wavelength surveys.We predict that these
highly-obscured sources are characterized by a steep spectrum in the infrared (1.6−4.5μm),
i.e. by very red colors. The method selects the only 2 objects with a robust X-ray detection
found in the CANDELS/GOODS-S survey with a photometric redshift z & 6. Fitting their
infrared spectra only with a stellar component would require unrealistic star formation rates
(& 2000M# yr−1). To date, the selected objects represent the most promising black hole seed
candidates, possibly formed via the direct collapse black hole scenario, with predicted mass
> 105M#. While this result is based on the best photometric observations of high-z sources
available to date, additional progress is expected from spectroscopic and deeper X-ray data.
Upcoming observatories, like the JWST, will greatly expand the scope of this work.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
Exocometary gas in_th_hd_181327_debris_ringSérgio Sacani
An increasing number of observations have shown that gaseous debris discs are not an
exception. However, until now we only knew of cases around A stars. Here we present the first
detection of 12CO (2-1) disc emission around an F star, HD 181327, obtained with ALMA
observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric
disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with
radiative transfer calculations we study the dust and CO mass distribution. We find the dust is
distributed in a ring with a radius of 86:0 0:4 AU and a radial width of 23:2 1:0 AU. At
this frequency the ring radius is smaller than in the optical, revealing grain size segregation
expected due to radiation pressure. We also report on the detection of low level continuum
emission beyond the main ring out to 200 AU. We model the CO emission in the non-LTE
regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging
between 1:2 10 6 M and 2:9 10 6 M, depending on the gas kinetic temperature and
collisional partners densities. The CO densities and location suggest a secondary origin, i.e.
released from icy planetesimals in the ring. We derive a CO cometary composition that is
consistent with Solar system comets. Due to the low gas densities it is unlikely that the gas is
shaping the dust distribution.
The puzzling source_in_ngc6388_a_possible_planetary_tidal_disruption_eventSérgio Sacani
Artigo descreve a descoberta da destruição de um planeta ao passar próximo a uma estrela do tipo anã branca presente dentro do aglomerado globular de estrelas NGC 6388. Para isso os astrônomos utilizaram um arsenal de telescópios.
Stellar-like objects with effective temperatures of 2700K and below are referred to as
20 "ultracool dwarfs"1. This heterogeneous group includes both extremely low-mass stars
21 and brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion),
22 and represents about 15% of the stellar-like objects in the vicinity of the Sun2. Based on
23 the small masses and sizes of their protoplanetary disks3,4, core-accretion theory for
24 ultracool dwarfs predicts a large, but heretofore undetected, population of close-in
25 terrestrial planets5, ranging from metal-rich Mercury-sized planets6 to more hospitable
26 volatile-rich Earth-sized planets7. Here we report the discovery of three short-period
27 Earth-sized planets transiting an ultracool dwarf star 12 parsecs away. The inner two
28 planets receive four and two times the irradiation of Earth, respectively, placing them
29 close to the inner edge of the habitable zone of the star8. Eleven orbits remain possible
30 for the third planet based on our data, the most likely resulting in an irradiation
31 significantly smaller than Earth's. The infrared brightness of the host star combined
32 with its Jupiter-like size offer the possibility of constraining the composition and
33 thoroughly characterizing the atmospheric properties of the components of this nearby
34 planetary system, notably to detect potential biosignatures.
Evidence for the_thermal_sunyaev-zeldovich_effect_associated_with_quasar_feed...Sérgio Sacani
Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar
catalogue, spanning redshifts 0.5–3.5, we derive the mean millimetre and far-infrared
quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology
Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We
constrain the form of the far-infrared emission and find 3σ–4σ evidence for the thermal
Sunyaev-Zel’dovich (SZ) effect, characteristic of a hot ionized gas component with
thermal energy (6.2 ± 1.7) × 1060 erg. This amount of thermal energy is greater than
expected assuming only hot gas in virial equilibrium with the dark matter haloes of
(1 − 5) × 1012h
−1M that these systems are expected to occupy, though the highest
quasar mass estimates found in the literature could explain a large fraction of this
energy. Our measurements are consistent with quasars depositing up to (14.5±3.3) τ
−1
8
per cent of their radiative energy into their circumgalactic environment if their typical
period of quasar activity is τ8 × 108 yr. For high quasar host masses, ∼ 1013h
−1M,
this percentage will be reduced. Furthermore, the uncertainty on this percentage is
only statistical and additional systematic uncertainties enter at the 40 per cent level.
The SEDs are dust dominated in all bands and we consider various models for dust
emission. While sufficiently complex dust models can obviate the SZ effect, the SZ
interpretation remains favoured at the 3σ–4σ level for most models.
“Nuevas vías para cubrir las necesidades sanitarias y sociales con soluciones tecnológicas efectivas” (en ingles, “New Avenues to fill the gap between Health and Welfare Demands and Effective ICT Solutions“). Su objetivo principal es analizar la falta de sintonía entre las necesidades públicas y la introducción de desarrollos tecnológicos con el fin de impulsar la inversión y permitir el despliegue a gran escala de soluciones AAL (Ambient Assisted Living –Vida Asistida Independiente-).
The Core Passion Factor Business Mastery training program is designed for business owners with a desire to enhance their business success using the fusion of human energy and business energy to generate profits.
Exocometary gas in_th_hd_181327_debris_ringSérgio Sacani
An increasing number of observations have shown that gaseous debris discs are not an
exception. However, until now we only knew of cases around A stars. Here we present the first
detection of 12CO (2-1) disc emission around an F star, HD 181327, obtained with ALMA
observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric
disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with
radiative transfer calculations we study the dust and CO mass distribution. We find the dust is
distributed in a ring with a radius of 86:0 0:4 AU and a radial width of 23:2 1:0 AU. At
this frequency the ring radius is smaller than in the optical, revealing grain size segregation
expected due to radiation pressure. We also report on the detection of low level continuum
emission beyond the main ring out to 200 AU. We model the CO emission in the non-LTE
regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging
between 1:2 10 6 M and 2:9 10 6 M, depending on the gas kinetic temperature and
collisional partners densities. The CO densities and location suggest a secondary origin, i.e.
released from icy planetesimals in the ring. We derive a CO cometary composition that is
consistent with Solar system comets. Due to the low gas densities it is unlikely that the gas is
shaping the dust distribution.
The puzzling source_in_ngc6388_a_possible_planetary_tidal_disruption_eventSérgio Sacani
Artigo descreve a descoberta da destruição de um planeta ao passar próximo a uma estrela do tipo anã branca presente dentro do aglomerado globular de estrelas NGC 6388. Para isso os astrônomos utilizaram um arsenal de telescópios.
Stellar-like objects with effective temperatures of 2700K and below are referred to as
20 "ultracool dwarfs"1. This heterogeneous group includes both extremely low-mass stars
21 and brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion),
22 and represents about 15% of the stellar-like objects in the vicinity of the Sun2. Based on
23 the small masses and sizes of their protoplanetary disks3,4, core-accretion theory for
24 ultracool dwarfs predicts a large, but heretofore undetected, population of close-in
25 terrestrial planets5, ranging from metal-rich Mercury-sized planets6 to more hospitable
26 volatile-rich Earth-sized planets7. Here we report the discovery of three short-period
27 Earth-sized planets transiting an ultracool dwarf star 12 parsecs away. The inner two
28 planets receive four and two times the irradiation of Earth, respectively, placing them
29 close to the inner edge of the habitable zone of the star8. Eleven orbits remain possible
30 for the third planet based on our data, the most likely resulting in an irradiation
31 significantly smaller than Earth's. The infrared brightness of the host star combined
32 with its Jupiter-like size offer the possibility of constraining the composition and
33 thoroughly characterizing the atmospheric properties of the components of this nearby
34 planetary system, notably to detect potential biosignatures.
Evidence for the_thermal_sunyaev-zeldovich_effect_associated_with_quasar_feed...Sérgio Sacani
Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar
catalogue, spanning redshifts 0.5–3.5, we derive the mean millimetre and far-infrared
quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology
Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We
constrain the form of the far-infrared emission and find 3σ–4σ evidence for the thermal
Sunyaev-Zel’dovich (SZ) effect, characteristic of a hot ionized gas component with
thermal energy (6.2 ± 1.7) × 1060 erg. This amount of thermal energy is greater than
expected assuming only hot gas in virial equilibrium with the dark matter haloes of
(1 − 5) × 1012h
−1M that these systems are expected to occupy, though the highest
quasar mass estimates found in the literature could explain a large fraction of this
energy. Our measurements are consistent with quasars depositing up to (14.5±3.3) τ
−1
8
per cent of their radiative energy into their circumgalactic environment if their typical
period of quasar activity is τ8 × 108 yr. For high quasar host masses, ∼ 1013h
−1M,
this percentage will be reduced. Furthermore, the uncertainty on this percentage is
only statistical and additional systematic uncertainties enter at the 40 per cent level.
The SEDs are dust dominated in all bands and we consider various models for dust
emission. While sufficiently complex dust models can obviate the SZ effect, the SZ
interpretation remains favoured at the 3σ–4σ level for most models.
“Nuevas vías para cubrir las necesidades sanitarias y sociales con soluciones tecnológicas efectivas” (en ingles, “New Avenues to fill the gap between Health and Welfare Demands and Effective ICT Solutions“). Su objetivo principal es analizar la falta de sintonía entre las necesidades públicas y la introducción de desarrollos tecnológicos con el fin de impulsar la inversión y permitir el despliegue a gran escala de soluciones AAL (Ambient Assisted Living –Vida Asistida Independiente-).
The Core Passion Factor Business Mastery training program is designed for business owners with a desire to enhance their business success using the fusion of human energy and business energy to generate profits.
In the field of high quality and stringent technical lighting design, SONARAY is one of the most vibrant brands of LED lighting products. Built upon a combination of constructive thought process and superior engineering, we have developed into a specialist in the industry of mid to high-end indoor and outdoor lighting.
The evolution of b2 b sales in the digital era handoutRoger Hage
This is a Key Note Speech I held at the FH OÖ – University of Applied Sciences Upper Austria
School of Management, during the International Days on May 18, 2015.
Business-to-Business (B2B) sales of technology and investment goods is complex, involves many decision makers & influencers, is very solution-oriented and requires different sales strategies, techniques and skills than transactional Business-to-Consumer (B2C) sales.
On the other hand, the buyers are becoming more educated & informed than ever before and are nearly 60% through their purchase process before their first contact with a supplier, rendering the commonly used solution selling and consultative selling approaches sometimes irrelevant.
How can you adjust your sales process to the new realities and enable your sales force with new skills and tools?
Uma espetacular colisão de galáxias foi descoberta além da Via Láctea. O sistema mais próximo já descoberto, a identificação foi anunciada por uma equipe de astrônomos liderada pelo Professor Quentin Parker da Universidade de Hong Kong e pelo Professor Albert Zijlstra na Universidade de Manchester.
A galáxia está a 30 milhões de anos-luz de distância, o que significa que ela é relativamente próxima. Ela foi chamada de Roda de Kathryn, em homenagem à sua semelhança com o famoso fogo de artifício e também em homenagem à esposa do coautor do trabalho.
Esses sistemas são muito raros e nascem da colisão entre duas galáxias de tamanhos similares. As ondas de choque geradas na colisão comprimem o reservatório de gás em cada galáxia e disparam a formação de novas estrelas. Isso cria um espetacular anel de intensa emissão, e ilumina o sistema, do mesmo modo que a Roda Catherine ilumina a noite num show de fogos de artifício.
As galáxias crescem através de colisões, mas é raro registrar esse processo acontecendo, e é extremamente raro ver o anel da colisão em progresso. Pouco mais de 20 sistemas com anéis completos são conhecidos.
Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents,
including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter (DM) and
the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio
Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields
cluster MACS J0416.1-2403 (z = 0.396), which consists of NE and SW subclusters whose cores are separated on
the sky by ∼250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a
cool core. Approximately 450 kpc south–southwest of the SW subcluster, we detect a density discontinuity that
corresponds to a compression factor of ∼1.5. The discontinuity was most likely caused by the interaction of the
SW subcluster with a less massive structure detected in the lensing maps SW of the subclusterʼs center. For both
the NE and the SW subclusters, the DM and the gas components are well-aligned, suggesting that MACS J0416.1-
2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The
halo has a 1.4 GHz power of (1.3 ± 0.3) × 1024WHz−1, which is somewhat lower than expected based on the
X-ray luminosity of the cluster if the spectrum of the halo is not ultra-steep. We suggest that we are either
witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.
Spirals and clumps in V960 Mon: signs of planet formation via gravitational i...Sérgio Sacani
The formation of giant planets has traditionally been divided into two pathways: core accretion and gravitational instability. However, in recent years, gravitational instability has become less favored, primarily due
to the scarcity of observations of fragmented protoplanetary disks around young stars and low occurrence rate
of massive planets on very wide orbits. In this study, we present a SPHERE/IRDIS polarized light observation
of the young outbursting object V960 Mon. The image reveals a vast structure of intricately shaped scattered
light with several spiral arms. This finding motivated a re-analysis of archival ALMA 1.3 mm data acquired
just two years after the onset of the outburst of V960 Mon. In these data, we discover several clumps of continuum emission aligned along a spiral arm that coincides with the scattered light structure. We interpret the
localized emission as fragments formed from a spiral arm under gravitational collapse. Estimating the mass of
solids within these clumps to be of several Earth masses, we suggest this observation to be the first evidence of
gravitational instability occurring on planetary scales. This study discusses the significance of this finding for
planet formation and its potential connection with the outbursting state of V960 Mon.
The Expansion of the X-Ray Nebula Around η CarSérgio Sacani
The massive colliding wind binary system η Car is embedded in an X-ray emitting region having a characteristic
temperature of a few million degrees, associated with ejecta produced during the 1840s, and in earlier outbursts.
We use CHANDRA X-ray imaging observations obtained over the past two decades to directly measure the
expansion of the X-ray nebula for the first time. A combined CHANDRA/ACIS image shows a faint, nearly
uniform elliptic structure. This faint elliptical “shell” has a similar orientation and shape as the Homunculus nebula
but is about 3 times larger. We measure proper motions of brighter regions associated with the X-ray emitting ring.
We compare spectra of the soft X-ray emitting plasma in CHANDRA/ACIS and XMM-Newton PN observations
and show that the PN observations indicate a decline in X-ray flux which is comparable to that derived from
NICER observations. We associate the diffuse elliptical emission surrounding the bright X-ray “ring” with the blast
wave produced during the Great Eruption. We suggest that the interaction of this blast wave with pre-existing
clumps of ejecta produces the bright, broken X-ray emitting ring. We extrapolate the trend in X-ray energy back to
the time of the Great Eruption using a simple model and show that the X-ray energy was comparable to the kinetic
energy of the Homunculus, suggesting equipartition of energy between fast, low-density ejecta and slower, dense
ejecta.
Triple Spiral Arms of a Triple Protostar System Imaged in Molecular LinesSérgio Sacani
Most stars form in multiple-star systems. For a better understanding of their formation processes, it is important to
resolve the individual protostellar components and the surrounding envelope and disk material at the earliest
possible formation epoch, because the formation history can be lost in a few orbital timescales. Here we present
Atacama Large Millimeter/submillimeter Array observational results of a young multiple protostellar system,
IRAS 04239+2436, where three well-developed large spiral arms were detected in the shocked SO emission.
Along the most conspicuous arm, the accretion streamer was also detected in the SO2 emission. The observational
results are complemented by numerical magnetohydrodynamic simulations, where those large arms only appear in
magnetically weakened clouds. Numerical simulations also suggest that the large triple spiral arms are the result of
gravitational interactions between compact triple protostars and the turbulent infalling envelope.
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.
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.
Chiotelis Ioannis, Theodoropoulou Maria, “Searching for Black Holes. Photometry in our Classrooms”, Hellenic Conference on Innovating STEM Education, 16-18 December 2016, Athens, Greece.
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.
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.
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.
X-rays from a Central “Exhaust Vent” of the Galactic Center ChimneySérgio Sacani
Using deep archival observations from the Chandra X-ray Observatory, we present an analysis of
linear X-ray-emitting features located within the southern portion of the Galactic center chimney,
and oriented orthogonal to the Galactic plane, centered at coordinates l = 0.08◦
, b = −1.42◦
. The
surface brightness and hardness ratio patterns are suggestive of a cylindrical morphology which may
have been produced by a plasma outflow channel extending from the Galactic center. Our fits of the
feature’s spectra favor a complex two-component model consisting of thermal and recombining plasma
components, possibly a sign of shock compression or heating of the interstellar medium by outflowing
material. Assuming a recombining plasma scenario, we further estimate the cooling timescale of this
plasma to be on the order of a few hundred to thousands of years, leading us to speculate that a
sequence of accretion events onto the Galactic Black Hole may be a plausible quasi-continuous energy
source to sustain the observed morphology
X-rays from a Central “Exhaust Vent” of the Galactic Center Chimney
Creation of cosmic structure in the complex galaxy cluster merger abell 2744
1. Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 16 June 2011 (MN L TEX style file v2.2)
A
Creation of cosmic structure in the complex galaxy cluster
merger Abell 2744
J. Merten,1,2 † D. Coe,3 R. Dupke,4,5,6 R. Massey,7 A. Zitrin,8 E. S. Cypriano,9
N. Okabe,10 B. Frye,11 F. Braglia,12 Y. Jim´nez-Teja,13 N. Ben´ 13
e ıtez,
14,15 16,17
T. Broadhurst, J. Rhodes, M. Meneghetti, L. A. Moustakas,17 L. Sodr´ Jr.,9
2
e
18 4
J. Krick and J. N. Bregman
1 Institut f¨r Theoretische Astrophysik, ZAH, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
u
2 INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
3 Space Telescope Science Institute, Baltimore, MD 21218, USA
4 University of Michigan, Ann Arbor, MI 48109-1090, USA
5 Eureka Scientific, 2452 Delmer Street 100, Oakland CA, USA
6 Observat´rio Nacional, Rua General Jos´ Cristino 77, Rio de Janeiro, 20921-400, Brazil
o e
7 University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
8 School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
9 Instituto de Astronomia, Geof´ ısica e Ciˆncias Atmosfericos, Universidade de S˜o Paulo (IAG/USP), 05508-090 S˜o Paulo/S.P.,Brazil
e a a
10 Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), P.O. Box 23-141, Taipei 10617, Taiwan
(Additional affiliations after the conclusions)
16 June 2011
ABSTRACT
We present a detailed strong lensing, weak lensing and X-ray analysis of Abell 2744
(z = 0.308), one of the most actively merging galaxy clusters known. It appears to
have unleashed ‘dark’, ‘ghost’, ‘bullet’ and ‘stripped’ substructures, each ∼ 1014 M .
The phenomenology is complex and will present a challenge for numerical simulations
to reproduce. With new, multiband HST imaging, we identify 34 strongly-lensed im-
ages of 11 galaxies around the massive Southern ‘core’. Combining this with weak
lensing data from HST, VLT and Subaru, we produce the most detailed mass map
of this cluster to date. We also perform an independent analysis of archival Chan-
dra X-ray imaging. Our analyses support a recent claim that the Southern core and
Northwestern substructure are post-merger and exhibit morphology similar to the
Bullet Cluster viewed from an angle. From the separation between X-ray emitting gas
and lensing mass in the Southern core, we derive a new and independent constraint
on the self-interaction cross section of dark matter particles σ/m < 3 ± 1 cm2 /g. In
the Northwestern substructure, the gas, dark matter, and galaxy components have
become separated by much larger distances. Most curiously, the ‘ghost’ clump (pri-
marily gas) leads the ‘dark’ clump (primarily dark matter) by more than 150 kpc. We
propose an enhanced ‘ram-pressure slingshot’ scenario which may have yielded this
reversal of components with such a large separation, but needs further confirmation
by follow-up observations and numerical simulations. A secondary merger involves a
second ‘bullet’ clump in the North and an extremely ‘stripped’ clump to the West.
The latter appears to exhibit the largest separation between dark matter and X-ray
emitting baryons detected to date in our sky.
Key words: Dark matter — Gravitational lensing: strong — Gravitational lensing:
weak — X-rays: galaxies: clusters — Galaxies: clusters: individual: Abell 2744.
1 INTRODUCTION
The standard ΛCDM cosmological model suggests a bottom-
Both authors contributed equally to this work up sequence of structure formation, in which a series of
† E-mail: jmerten@ita.uni-heidelberg.de merging events culminates in massive clusters of galaxies,
c 0000 RAS
2. 2 J. Merten et al.
the latest structures to form in the observable Universe high Mach shocks or turbulence (e.g. Sarazin 2004). The
(Bond et al. 1991; Lacey & Cole 1993). The number of clus- picture was clarified by X-ray studies (Kempner & David
ters as a function of their mass (the steep, high end of the 2004; Zhang et al. 2004) that revealed substructure near the
mass function) depends sensitively upon cosmological pa- cluster core, plus an additional luminous structure towards
rameters (e.g. Vikhlinin et al. 2009) and has become an im- the Northwest. Kinematic observations of cluster member
portant observational test of cosmology. Most measurements galaxies (Girardi & Mezzetti 2001) suggested a bimodal dis-
of cluster masses rely upon the calibration of more easily ob- tribution in redshift space, but were not at first consid-
servable proxies, such as X-ray luminosity, temperature or ered significant. Recent kinematic studies focussing solely
galaxy richness. However, clusters form through multiple, on Abell 2744 definitely show a bimodal velocity dispersion
dynamic accretions, so are likely to be turbulent places, and in the cluster centre, together with a third group of cluster
the turmoil affects those observable proxies. It is therefore members near the Northwestern X-ray peak (Boschin et al.
vital to quantitatively understand the merging process, for 2006; Braglia et al. 2009). Although there is no evidence for
example by mapping the distribution of dark matter, stars non-thermal X-ray emission (Million & Allen 2009), the frac-
and baryonic gas in systems at many different stages of the tion of blue star-forming galaxies (Braglia et al. 2007, and
merger. references therein) also seems to be enhanced. A default ex-
Merging clusters of galaxies have become useful labo- planation emerged for the centre of Abell 2744, featuring a
ratories in which to study the nature and interaction prop- major merger in the North-South direction with a small in-
erties of dark matter. The best-studied example is the Bul- clination towards the line-of-sight and a ∼ 3:1 mass ratio of
let Cluster 1ES 0657-558 (z = 0.296) (Tucker et al. 1998; the merging entities (Kempner & David 2004; Boschin et al.
Markevitch et al. 2002). Combined X-ray and gravitational 2006). More controversial is the role of the Northwestern
lensing analyses show a clear separation between the centres structure. Kempner & David (2004) detected a cold front
of X-ray emission and the peaks in surface mass density – on its SW edge and a possible shock front towards the clus-
indicating a fundamental difference between baryonic gas, ter core, so concluded that it is falling towards the main
which feels the pressure of the collision, and dark matter, mass. More recent analysis of Chandra data (Owers et al.
which is nearly collisionless (Clowe et al. 2004, 2006; Bradaˇ
c 2011) failed to confirm the presence of the shock front, only
et al. 2006). The discovery and interpretation of the Bul- a cold front towards the Northern edge, and the authors
let Cluster has inspired a lively debate about whether such proposed that the structure is moving instead towards the
a system could exist in different cosmological models (see Northwest, away from the main cluster. What seems sure is
e.g. Hayashi & White 2006). Improvements are continuing that we are at least observing a complicated merger between
in numerical simulations (Milosavljevi´ et al. 2007; Springel
c three separate bodies (Braglia et al. 2007).
& Farrar 2007; Mastropietro & Burkert 2008; Lee & Ko-
matsu 2010). Observations have also broadened, with discov-
eries of a possible line-of-sight merger CL0024+1654 (Czoske
Until now, Abell 2744 has been better constrained from
et al. 2002; Hoekstra 2007; Jee et al. 2007; Zitrin et al.
X-ray and kinematic studies than by gravitational lensing.
2009; Umetsu et al. 2010; Zu Hone et al. 2009,a), and other
So far, Smail et al. (1997) detected a weak-lensing signal and
systems including the Baby Bullet (MACS J0025.4-1222)
strong-lensing features, followed by Allen (1998) who found
(Bradaˇ et al. 2008), the Cosmic Train Wreck (Abell 520)
c
a large discrepancy in the mass estimates for Abell 2744
(Mahdavi et al. 2007; Okabe & Umetsu 2008), Abell 2146
from X-ray and strong-lensing reconstructions. Given the
(Russell et al. 2010), Abell 521 (Giacintucci et al. 2008; Ok-
cluster’s dynamical state, this finding is perhaps no longer
abe et al. 2010a) and Abell 3667 (Finoguenov et al. 2010).
surprising since the merging would induce non-thermal sup-
All these systems place potentially tight constraints on the
port and elongation along the line-of-sight, which increases
interaction between baryons and dark matter, and are ex-
the systematics from both methods. The most recent weak-
emplary probes for our understanding of structure formation
lensing analysis (Cypriano et al. 2004) showed indications of
within gravitationally bound systems.
substructure in the reconstructed surface-mass density, but
In the archival data of 38 merging clusters, Shan et al.
did not reach the resolution required for more quantitative
(2010) found the largest offset between X-ray and lensing
statements. Here, we present the results of an HST imag-
signals to occur in the massive (LX = 3.1×1045 erg/s in the
ing survey, aimed at clarifying the evolutionary stage of this
2-10 keV range, Allen (1998)) cluster Abell 2744 (also known
complex system.
as AC118, or RXCJ0014.3-3022) at a redshift of z = 0.308
(Couch & Newell 1984). The ∼ 250 kpc offset is larger than
that in the Bullet Cluster – although, as we shall discuss
later, this value does not describe the separation of the This article is organised as follows. In Sec. 2 we present
main mass-clump from its stripped gas component. Nev- our comprehensive lensing analysis, mainly based on newly
ertheless, Abell 2744 is undergoing a particularly interest- acquired data taken with the Advanced Camera for Surveys
ing merger. The complex interplay between multiple dark (ACS) on the Hubble Space Telescope (HST). In Sec. 3 we
matter and baryonic components appears to have unleashed describe a complementary X-ray analysis based on Chandra
‘ghost’, ‘dark’, ‘stripped’ and ‘bullet’ clusters. data. We discuss cosmological implications and an interpre-
The first hint that Abell 2744 is in the middle of a ma- tation of the merging scenario in Sec. 4, and we conclude
jor merging event arose from observation of a powerful and in Sec. 5. Throughout this paper we assume a cosmologi-
extended radio halo (P (1.4GHz) > 1.6 × 1036 Watt, Gio- cal model with Ωm = 0.3, ΩΛ = 0.7 and h = 0.7. At the
vannini et al. (1999); Govoni et al. (2001,a)). This indicated cluster’s redshift z = 0.308, one arcsecond corresponds to
the presence of relativistic electrons accelerated through 4.536 kpc.
c 0000 RAS, MNRAS 000, 000–000
3. The galaxy cluster merger Abell 2744 3
2 LENSING ANALYSIS trailing and correct the raw images pixel-by-pixel, we used
the detector readout model of Massey et al. (2010), updated
Our recently acquired, multiband HST/ACS imaging en-
for device performance post Servicing Mission 4 by Massey
ables us to significantly improve upon previous mass mod-
(2010). The corrected data was then reduced via the stan-
els of Abell 2744. Using the parametric method described in
dard CALACS pipeline (Pavlovsky 2006), and stacked using
Sec. 2.2, we have identified strong gravitational lensing of 11
multidrizzle (Koekemoer et al. 2002).
background galaxies producing 34 multiple images around
the Southern core, with an Einstein radius of rE ∼ 30 (see
below). These enable us to tightly constrain the position and
2.2 Strong lensing
shape of the core mass distribution. No such multiple image
systems are revealed around the N or NW clumps, immedi- We concentrate our strong-lensing analysis on the reduced
ately indicating that their masses are lower. Our HST im- ACS images only. Several strong-lensing features are imme-
ages also yield ∼ 62 galaxies/arcmin2 for weak lensing anal- diately identifiable by eye on the combined three-band image
ysis (after charge transfer inefficiency (CTI) corrections are (Fig. 2). To find additional multiple images across the field
performed, as described below). This enables detailed mass of view, we apply the well-tested approach of Zitrin et al.
modelling throughout our HST field of view. To probe the (2009) (see also Zitrin et al. 2011b) to lens modelling, which
cluster merger on even larger scales, we incorporate ground- has previously uncovered large numbers of multiply-lensed
based weak lensing measurements from VLT and Subaru. galaxies in ACS images of Abell 1689, Cl0024, 12 high-z
We simultaneously fit all of these strong- and weak- MACS clusters, MS1358 and Abell 383 (respectively, Broad-
lensing observations using our well-tested mass reconstruc- hurst et al. 2005; Zitrin et al. 2009, 2011, 2010, 2011a,c) .
tion algorithm (Merten et al. 2009; Meneghetti et al. 2010) In the Zitrin et al. (2009) method, the large-scale distri-
(which is similar to that used by Bradaˇ et al. (2006) and
c bution of cluster mass is approximated by assigning a power-
Bradaˇ et al. (2008) to map the Bullet Cluster and the Baby
c law mass profile to each galaxy, the sum of which is then
Bullet). Importantly, we make no assumptions about mass smoothed. The degree of smoothing (S) and the index of
tracing light in the combined analysis. It should be noted the power-law (q) are the most important free parameters
though, that we rely on a parametric method, assuming that determining the mass profile. A worthwhile improvement in
light traces mass, to identify multiple image systems. We fitting the location of the lensed images is generally found
account for possible uncertainties in this identification, by by expanding to first order the gravitational potential of this
resampling the set of less confident identifications in the er- smooth component, equivalent to a coherent shear describ-
rors analysis based on a bootstrapping technique (see Sec. ing the overall matter ellipticity, where the direction of the
2.4). Our analysis reveals four individual clumps of mass shear and its amplitude are free parameters. This allows for
> 14
∼ 10 M within a 250 kpc radius. Previous weak lensing some flexibility in the relation between the distribution of
analysis of VLT images alone had resolved but a single broad dark matter and the distribution of galaxies, which cannot
mass clump (Cypriano et al. 2004). Below we describe our be expected to trace each other in detail. The total deflec-
datasets, analyses, and results in more detail. The central tion field αT (θ), consists of the galaxy component, αgal (θ),
cluster field and a preview on the matter and gas distribu- scaled by a factor Kgal , the cluster dark matter component
tion is presented in Fig. 1, which also shows the contours αDM (θ), scaled by (1-Kgal ), and the external shear compo-
encircling different areas of specific probability, that a mass nent αex (θ):
peak is located at this specific location of the reconstructed
field. αT (θ) = Kgal αgal (θ) + (1 − Kgal )αDM (θ) + αex (θ), (1)
where the deflection field at position θm due to the external
shear, αex (θm ) = (αex,x , αex,y ), is given by:
2.1 The HST/ACS dataset
The HST data consist of two pointings in Cycle 17 αex,x (θm ) = |γ| cos(2φγ )∆xm + |γ| sin(2φγ )∆ym , (2)
(data taken between Oct. 27-30 2009, Proposal ID: 11689, αex,y (θm ) = |γ| sin(2φγ )∆xm − |γ| cos(2φγ )∆ym , (3)
P.I.: R. Dupke) with ∼ 50% overlap between the point-
ings. The images were taken with the ACS/WFC camera where (∆xm , ∆ym ) is the displacement vector of the posi-
using three different filters, F435W (16.2 ksec1 ), F606W tion θm with respect to a fiducial reference position, which
(13.3 ksec) and F814W (13.2 ksec). we take as the lower-left pixel position (1, 1), and φγ is the
The HST/ACS camera had been in orbit for eight years position angle of the spin-2 external gravitational shear mea-
when the imaging was acquired. During this time above sured anti-clockwise from the x-axis. The normalisation of
the protection of the Earth’s atmosphere, its CCD detec- the model and the relative scaling of the smooth dark matter
tors had been irreparably damaged by a bombardment of component versus the galaxy contribution brings the total
high energy particles. During CCD readout, photoelectrons number of free parameters in the model to 6. This approach
are transported to the readout amplifier through a silicon to strong lensing is sufficient to accurately predict the lo-
lattice. Damage to this lattice creates charge traps that de- cations and internal structure of multiple images, since in
lay some electrons and spuriously trail the image – in a way practice the number of multiple images readily exceeds the
that alters the shapes of galaxies more than the gravitational number of free parameters, which become fully constrained.
lensing signal that we are trying to measure. To undo this Two of the 6 free parameters, namely the galaxy power
law index q and the smoothing degree S, can be initially set
to reasonable values so that only 4 of the free parameters
1 Equally split between the two pointings need to be fitted at first. This sets a very reliable starting-
c 0000 RAS, MNRAS 000, 000–000
4. 4 J. Merten et al.
Figure 1. The field of Abell 2744, with different interesting features. The false-colour background is provided by HST/ACS (the two
pointings can be identified by the higher, bluely background noise level), VLT and Subaru images on a field size of 240 × 240 (∼ 1.1
Mpc on a side). Overlaid in cyan are the surface-mass density contours most concentrated in the ’core’ area and in magenta the more
evenly-distributed X-ray luminosity contours. The peak positions of the core, N, NW, and W clumps are indicated by the green likelihood
contours, derived from the bootstrap samples. Contours are 86%, 61%, and 37% of the peak likelihood for each clump. (Assuming a
Gaussian probability distribution, these would correspond to 0.3, 1, and 2-sigma confidence contours.) Note that the NW clump most
likely peaks in the area indicated by NW1 (in 95% of the bootstrap realisations), but peaks also at NW2 in 54% of the bootstrap
realisations. The small red circles show the position of the local overdensities in the gas distribution, associated with each individual dark
matter clump. The white rulers show the separation between dark matter peaks and the bright clusters galaxies and local gas peaks.
2
X
point using obvious systems. The mass distribution is there- RM Simages = ((xi − xi )2 + (yi − yi )2 ) / Nimages , (4)
fore well-constrained and uncovers many multiple-images i
that can be iteratively incorporated into the model, by using
where xi and yi are the locations given by the model, xi
their redshift estimation, from photometry, spectroscopy or
and yi are the real image locations, and the sum is over
model prediction and location in the image-plane. At each
the total number of images Nimages . The best-fit solution
stage of the iteration, we use the model to lens the most ob-
is unique in this context, and the model uncertainty is de-
vious lensed galaxies back to the source plane by subtracting
termined by the locations of predicted images in the image
the derived deflection field, then relens the source plane to
plane. Importantly, this image-plane minimisation does not
predict the detailed appearance and location of additional
suffer from the well known bias involved with source plane
counter images, which may then be identified in the data by
minimisation, where solutions are biased by minimal scatter
morphology, internal structure and colour. We stress that
towards shallow mass profiles with correspondingly higher
multiple images found this way must be accurately repro-
magnification.
duced by our model and are not simply eyeball “candidates”
In Abell 2744 we have uncovered a total number of 34
requiring redshift verification. The best fit is assessed by the
multiple images belonging to 11 background sources. We
minimum RMS uncertainty in the image plane
label the different systems in Fig. 2, which also shows the
critical curve of the cluster derived from the strong lensing
c 0000 RAS, MNRAS 000, 000–000
5. The galaxy cluster merger Abell 2744 5
Table 1. The multiple-image system of Abell 2744.
Image-ID x y z
( source . image additional knot ) ( ) ( )
1.1 −35.10 −13.55 2.0 ± 0.3
1.2 −30.15 −23.95
1.3 0.10 −35.40
1.11 −33.60 −16.50
1.21 −31.50 −21.55
1.31 1.60 −35.85
2.1 9.30 −11.50 2.0 ± 0.3
2.2 −34.25 12.35
2.3 2.80 1.05
(2.4) −0.50 −7.10
2.11 11.55 −7.65
2.21 −32.55 13.90
2.31 5.55 3.10
(2.41) −0.05 −4.15
3.1 −10.10 22.65 4.0 ± 0.3
3.2 −7.40 22.95
(3.3) 27.15 2.20
Figure 2. A zoom into the innermost core region of the
HST/ACS images. Shown as a continuous white line is the critical 4.1 −18.25 −9.00 3.5 ± 0.3
curve of the cluster as it is derived from the strong-lensing model. 4.2 −29.25 −5.30
It assumes a source redshift zs = 2.0. Also shown are the approx- 4.3 18.05 −31.60
imate positions of the identified multiple-image systems as they
5.1∗ 8.85 29.00 4.0 ± 0.5
are listed in Tab. 1 with a varying two-colour scheme to avoid
5.2∗ 3.85 31.60
confusion of close-by images. These systems were found by the
5.3∗ 19.65 19.30
method of Zitrin et al. (2009) and are not simple optical identi-
fications. The visible field size is ∼ 100 × 100 , translating to 6.1∗ −38.15 −5.95 3.0 ± 0.5
∼ 450 kpc on a side. 6.2∗ −24.25 −28.30
6.3∗ −0.60 −33.15
model. The model predicts an Einstein radius of rE = 30 ± 7.1∗ −37.35 −7.85 3.7 ± 0.5
7.2∗ −27.85 −26.10
3 . With such a large number of clear multiple images we
7.3∗ 5.10 −34.85
can constrain the inner mass distribution very well, and we
shall incorporate these constraints into our joint strong- and 8.1 −10.70 20.90 4.0 ± 0.2
weak-lensing analysis described in Sec. 2.4. 8.2 −8.00 21.40
(8.3) 16.10 5.90
9.1∗ −6.65 −18.45 3.0 ± 0.5
2.3 Weak lensing 9.2∗ −2.80 −21.90
(9.3) −43.20 10.80
Several areas outside the cluster core are of special interest,
due to the complicated structure of this merging cluster. 10.1∗ −6.65 −20.65 3.0 ± 0.5
We measure the shapes of background galaxies to derive a 10.2∗ −3.55 −22.75
weak-lensing signal and extend our mass reconstruction over 10.3∗ −44.90 11.00
a much larger field. Since our HST data cover only a limited (11.1) −16.00 −13.30 3.0 ± 0.5
field of view, we also include VLT and Subaru imaging in (11.2) −34.20 −4.65
our weak lensing analysis. Additional HST pointings in the (11.3) 10.70 −31.55
future are crucial for a full interpretation of this merger, as
we shall discuss in the course of this analysis.
If the image ID is shown in brackets, the image is not confi-
dently reproduced by the lensing model described in Sec. 2.2.
The systems marked with an asterisk, were nicely reproduced by
2.3.1 HST/ACS the parametric lensing model but we allow them to be resam-
To select lensed background galaxies, we obtained photomet- pled by our nonparametric reconstruction technique to allow for
uncertainties in the number of identified strong lensing features.
ric redshifts based on our three filters using BPZ (Ben´ ıtez
All other images define a ‘confident catalogue’ of multiple-image
2000; Coe et al. 2006). Cluster ellipticals at z ∼ 0.3 occupy
systems and are used in our subsequent analysis. The x-and y-
a unique region in our colour-colour space, enabling us to coordinates are relative to the BCG position (αJ2000 = 3.58611◦ ,
obtain better than expected results. δJ2000 = −30.40024◦ ) in arcseconds. Redshifts of each system
Of 118 galaxies with published spectroscopic redshifts and their respective error are derived from the model predictions.
(Owers et al. 2011 and references therein; all z < 0.7) within
our field of view, 99 yield confident photo-zs, accurate to
c 0000 RAS, MNRAS 000, 000–000
6. 6 J. Merten et al.
∆z ∼ 0.06(1 + z) RMS with no significant outliers. Back- 2.3.3 Subaru/SuprimeCam
ground galaxies were selected as those with confident photo-
To extend the total field of view even further, especially
z > 0.5. This cut successfully excludes all 5 foreground
in the Northern areas of the cluster field, we obtained 1.68
and 86 cluster galaxies in our spec-z sample with confident
ksec i -band imaging data with Subaru/SuprimeCam dur-
photo-z (though one cluster galaxy was assigned a less con-
ing Semester S08B. The data were reduced following Okabe
fident photo-z ∼ 0.8). However, given our limited photomet-
& Umetsu (2008) and Okabe et al. (2010,a). Astrometric
ric coverage, we do not rule out some contamination of our
calibration was conducted by fitting the final stacked image
background sample with foreground and/or cluster galaxies.
with the 2MASS data point source catalogue; residual as-
We measure the weak gravitational lensing signal in the
trometric errors were less than the CCD pixel size. Due to
F814W HST exposures, which are the deepest and contain
poor weather conditions, the seeing size is as large as 1. 28.
the most galaxies at high redshift. We measure their shapes,
We measure galaxy shapes and perform PSF correction
and correct them for convolution by the Point-Spread Func-
using the IMCAT package (provided by Kaiser et al. 1995) in
tion (PSF), using the ‘RRG’ (Rhodes et al. 2000) pipeline
the same pipeline as Okabe et al. (2010,a) with some modi-
developed for the HST COSMOS survey (Leauthaud et al.
fications following Erben et al. (2001). Background galax-
2007; Massey et al. 2007). The RRG method is particularly
ies were selected in the range of 22 < i < 26 ABmag
optimised for use on high resolution, space-based data. Since
¯∗
and rh + σrh ∗ 3.4 < rh < 6.0 pix, where rh is the
HST expands and contracts as it warms in the sun or passes
¯∗
half-light radius, and rh and σrh are the median and stan-
∗
through the shadow of the Earth, even this telescope does
dard error of stellar half-light radii rh∗ , corresponding to
not have a constant PSF. However, 97% of the variation in
the half median width of the circularised PSF. The den-
its PSF can be accounted for by variation in its focal length
sity of background galaxies in our final shear catalogue is
(Jee et al. 2007, the separation between the primary and
∼ 15 galaxies/arcmin2 . This is 30-50% of typical values from
secondary mirrors). We therefore measure its focal length
images obtained during normal weather conditions Okabe
by matching the shapes of the ∼ 12 bright stars in each
et al. (2010,a).
pointing to models created by raytracing through the opti-
cal design (Krist 2003). This achieves a repeatable precision
of 1µm in the determination of the focal length, and we con- 2.4 Combined lensing reconstruction
struct a PSF model from all those stars observed during the
600-orbit COSMOS survey at a similar focus (Rhodes et al. In order to combine the weak- and strong-lensing constraints
2007). We finally use this PSF model to correct the shapes in a consistent way, we use the joint lensing reconstruction
of galaxies, and to obtain estimates of the amount by which algorithm described in Merten et al. (2009) (see also Bradaˇ
c
their light has been sheared. The result is a catalogue of 1205 et al. 2005, 2009, for a similar approach). This method
galaxies with shear estimates, corresponding to a density of has been extensively tested in Meneghetti et al. (2010) and
∼ 62 galaxies/arcmin2 . proved its capability to faithfully recover the cluster mass
distribution over a broad range of scales.
Our joint mass reconstruction is nonparametric, in the
sense that it neither makes any a priori assumptions about
2.3.2 VLT/FORS1 the cluster’s underlying mass distribution nor does it need
The complementary VLT data for our weak lensing analysis to trace any light-emitting component in the observed field.
is identical to that included by Cypriano et al. (2004) in a However, as described in Sec. 2.2, we use a parametric
study of 24 X-ray Abell clusters. The total field of view is method to identify the multiple image systems in the field.
6. 8 on a side, centred on the cluster BCG and significantly We reconstruct the cluster’s lensing potential (its gravita-
exceeding the coverage of our HST imaging. V , R and I- tional potential projected onto the plane of the sky) ψ by
band imaging was obtained with the FORS1 camera between combining measurements of the position of the critical line
April and July 2001, with exposure times of 330 s in each and the reduced shear. To do this, we divide the observed
filter. The data were reduced with standard IRAF routines field into an adaptive mesh, which discretises all observed
and, to maximise the depth, we perform weak-lensing shape and reconstructed quantities. A statistical approach is cho-
analysis on the combined VRI image. Seeing conditions were sen to combine our various measurements, by defining a
excellent, with a FWHM of stars in the combined VRI image multi-component χ2 -function that depends on the underly-
of 0. 59. ing lensing potential and a regularisation term R(ψ) to pre-
We measure galaxy shapes and perform PSF correc- vent the reconstruction overfitting noise (see Merten et al.
tion using the IM2SHAPE method (Bridle et al. 2002). This 2009; Bradaˇ et al. 2005)
c
involves a two-step process to first map the PSF variation χ2 (ψ) = χ2 (ψ) + χ2 (ψ) + R(ψ).
w s (5)
across the observed field using stars, then to model each de-
tected galaxy, perform PSF correction and recover its ellip- The weak-lensing term is defined by the expectation
ticity. To remove foreground contamination and unreliable value of the complex reduced shear in each mesh position
shape measurements from our catalogue, we apply magni- ε , which is obtained by averaging the measured ellipticities
tude cuts plus additional rejection criteria (see Cypriano of all background galaxies within that grid cell
et al. 2004). Since that work, we have improved the effi- „
Z(z)γ(ψ)
« „
Z(z)γ(ψ)
«
−1
ciency of foreground galaxy removal and now keep a higher χ2 (ψ) =
w − Cij − , (6)
1 − Z(z)κ(ψ) i 1 − Z(z)κ(ψ) j
density of background galaxies in our shear catalogue. The
result is a catalogue of 912 galaxies with shear estimates, or where Z(z) is a cosmological weight factor as defined e.g. in
∼ 20 galaxies/arcmin2 . Bartelmann & Schneider (2001), and γ = ∂∂ψ/2 is the
c 0000 RAS, MNRAS 000, 000–000
7. The galaxy cluster merger Abell 2744 7
shear of the lens, with the two components expressed in VLT ellipticities in the innermost centre of the cluster is
complex notation. κ = ∂∂ ∗ ψ is the convergence, where problematic, but indeed the galaxy density is three times
the complex differential operator in the plane is defined as higher in the HST/ACS field and clearly dominating the
∂ ∂
∂ := ( ∂θ1 +i ∂θ2 ), with θ1 and θ2 being the two angular coor- adaptive-averaging process of the nonparametric reconstruc-
dinates in the sky. The indices i, j indicate the discretisation tion algorithm that we used for the further analysis (com-
of the input data and the lens properties, where we have to pare Sec. 2.4 and Merten et al. (2009)).
take into account the full χ2 -function because the averaging Another issue is the possible double-counting of galaxies, so
process of background galaxies might result in an overlap we cross-correlated both catalogues with a correlation ra-
of neighbouring mesh points, expressed by the covariance dius of 2.5 and found 160 double-count candidates over the
matrix Cij . full HST/ACS field. Given the pixel size of the final recon-
The strong-lensing term is defined as struction, this translates to an insignificant average double-
´2 count probability of less than one galaxy per pixel. Fur-
(1 − Z(z)κ(ψ))2 − |Z(z)γ(ψ)|2 k
`
2 (det A(ψ))2 k thermore, the weighting scheme of the adaptive-averaging
χs (ψ) = 2
= 2
, (7)
σs σs process was implemented such, that the highest weight of
where the index k labels all pixels in the reconstruction the VLT ellipticities was identical to the smallest weight of
mesh, which are supposed to be part of the critical curve the HST/ACS ellipticities. Finally, the errors in the ellip-
within the uncertainties σs , given by the pixel size of the ticity measurement are treated in the joint reconstruction
grid. At these points, the Jacobian determinant det A(ψ) of method in a purely statistical way by deriving the variance
the lens mapping must vanish. of the weight-averaged sample of ellipticities in each recon-
We iterate towards a best-fitting lens potential by min- struction pixel.
imising the χ2 -function at each mesh position To derive a physical surface mass density from the scaled
lensing convergence one needs to know at least the mean
∂χ2 (ψ) ! redshift of the background galaxy population that was used
=0 with l ∈ [0, ..., Npix ] . (8)
∂ψl to produce the ellipticity catalogues. Problems with the
In practice, we achieve this by translating this operation into different depths of the fields and therefore with the final
a linear system of equations and invoking a two-level itera- mass analysis should not be a crucial issue for a relatively
tion scheme (see Merten et al. 2009, and references therein). low-redshift cluster like Abell 2744. However, the redshift
In this analysis, we use all strongly lensed multiple- for determining the surface-mass density from the recon-
image systems from the confident sample (Tab. 1) together structed convergence in the overlap area has been calculated
with their derived redshifts, and combine weak lensing shear as the galaxy-density weighted average of both, the VLT
catalogues from all three telescopes (see Sec. 2.3) as de- and theHST/ACS populations. The redshifts of the strong
scribed in Sec. 2.4.1. The combination of strong lensing data lensing features (compare Tab. 1) were included for the de-
and the density of background galaxies allows for a recon- termination of the core mass and the respective redshifts
struction on a mesh of 72 × 72 pixels in the central region of the VLT and Subaru source distribution was used in the
(corresponding to a pixel-scale of 8.4 /pix) and 36×36 pixels outskirts of the field.
in the outskirts of the field (corresponding to a pixel-scale of In order to test the effect of dilution in the outskirts of
16.7 /pix). Error estimates were produced by bootstrapping the field, we increased the ellipticity values for all Subaru
the redshift uncertainties of the strong-lensing constraints, background galaxies by 15% and repeated the reconstruc-
and resampling those multiple image systems in Tab. 1, tion. This test is necessary due to the single-band Subaru
which are marked with an asterisk. The result are 500 dif- imaging. As it turns out, the difference in the reconstructed
ferent bootstrap realisations of the refined cluster core. 150 convergence is marginal since the ellipticity values are al-
bootstrap realisations of the cluster outskirts were produced ready low with large scatter in this area of the field. How-
by bootstrapping the combined ellipticity catalogues. The ever, the effect was included in the determination of the error
number of bootstrap realisations is mainly constrained by budget for the reconstructed total mass and mass profile.
runtime considerations. All error estimates are calculated
from the scatter within the full bootstrap sample. If not
stated differently, the given value reflects 68% confidence 2.5 Reconstruction results
level. We obtain a map of the lensing convergence across the field
(proportional to the projected mass) by applying the Lapla-
cian operator to the lensing potential on the adaptively re-
2.4.1 Combining the catalogues
fined mesh (see Fig. 3). We find a total mass (by assuming
In order to combine the three different catalogues of ellip- h = 0.7 here and further on) within a radius of 1.3 Mpc
ticity measurements we used the following strategy. Clearly, around the Core of M (r < 1.3Mpc) = 1.8 ± 0.4 × 1015 M ,
the Subaru catalogue delivered the largest field-of-view but which is in good agreement with kinematically derived
it was derived from only single-band imaging and under bad masses (Boschin et al. 2006). A mass determination within a
seeing conditions. Therefore, we decided to limit its field field of (1300×750) kpc (to compare easily to the work on the
size to a box of 600 × 600 around the centre of the cluster Bullet Cluster of Bradaˇ et al. (2006)) centred on the Core
c
and to cut out the central ∼ 400 × 400 part, which was density peak yields M (1.73Mpc2 ) = 7.4 ± 1.0 × 1014 M ,
sufficiently covered by HST/ACS and VLT exposures with rendering Abell 2744 comparable in mass or slightly less
better data quality. As a result, Subaru data only covers the massive than the Bullet Cluster (Bradaˇ et al. 2006). The
c
outermost 200 on each side of the field. overall radial convergence and mass profile can be found in
One might argue that the combination of HST/ACS and Fig. 4.
c 0000 RAS, MNRAS 000, 000–000
8. 8 J. Merten et al.
Table 2. Structures identified within our lensing reconstruction.
Name x y M (r < 250kpc)
( ) ( ) (1014 M )
Core −13+11
−8 −4+6
−13 2.24 ± 0.55
NW 71+11
−10 84+15
−7 1.15 ± 0.23
W 177+23
−17 −30+11
−15 1.11 ± 0.28
N 34+23
−32 170+10
−28 0.86 ± 0.22
The x-and y-coordinates are provided in arcseconds, relative to
the BCG position (αJ2000 = 3.58611◦ , δJ2000 = −30.40024◦ ).
The 68% confidence limits on peak positions are derived from 500
bootstrap realisations for the Core peak, and from the pixel size
of the coarse weak lensing mesh in the reconstruction outskirts for
the other mass peaks. Masses assume h = 0.7 and their 68% con-
fidence limits are derived from bootstrap realisations as described
in the text.
Figure 3. The convergence map of the cluster field for a source
redshift of zs = ∞ and a field size of 600 × 600 , translating
to ∼ 2.7 Mpc on a side. The black contours start at κ0 = 0.14
with a linear spacing of ∆κ = 0.047. The four white circles with
labels indicate identified sub-clumps and the radius within which
their mass is calculated. The radius of all four circles is 55.4 ≈
250 kpc.
Most interestingly, our gravitational lensing analysis re-
solves four distinct sub-structures, indicated in Fig. 3 by the
white circles. We label these substructures as Core, North-
western (NW, later on dubbed as ‘dark’), Western (W, later
on dubbed as ‘stripped’) and Northern (N) structure. Please
note that there is some indication of a possible double peak
in the NW structure, as can be seen in Fig. 1 and also in the
clearly visible extension of the structure towards the West
in Fig. 3. The Core, NW and W clumps are clear detections
in the surface-mass density distribution with 11σ, 4.9σ and
3.8σ significance over the background level, respectively. 2
Somewhat fainter with 2.3σ significance is the N structure, Figure 4. Radial mass profiles of the cluster, derived from our
but it clearly coincides with a prominent X-ray substructure lensing analysis. Shown in blue (referring to the left y-axis) is
found by Owers et al. (2011) and is therefore included in the the convergence profile for a source redshift zs = ∞. The large
further analysis (compare Sec. 3). The positions of the mass uncertainty for small radii arises from uncertainty in the exact
peaks and their local projected masses within 250 kpc are position of the cluster centre. The cumulative mass as a function
listed in Tab. 2 and shown in more detail in Fig. 1. The new of radial distance from the cluster centre is shown in red (referring
to the right y-axis). The black vertical line indicates the distance
HST/ACS images thus allow a striking improvement in our
of the outermost multiple images from the cluster centre.
map of the mass distribution and reveal the distribution of
dark matter sub-structure in great detail for the first time.
We will refer to the individual mass clumps resolved here in as its impact velocity, impact parameter and angle with re-
our discussion of the X-ray analysis below. spect to the plane of the sky (Markevitch et al. 2002). For
this purpose, X-ray data becomes a crucial addition to lens-
ing measurements. The location of any shock fronts are re-
3 X-RAY ANALYSIS vealed in the temperature of the intracluster medium (ICM).
Velocities can be inferred from the density and temperature
The most difficult part of interpreting merging clusters is de- of intracluster gas (if the merger axis is near the plane of
termining the geometric configuration of the collision, such the sky), or through direct Doppler measurements (if the
merger axis is near the line of sight).
2 The background level was estimated in the following way: From
the total convergence, shown in Fig. 3, a radial area of 75 around 3.1 Reduction of the Chandra data
the Core, 70 around NW and 55 around N and W was cut out.
From the remaining field the mean and variance in the conver- We reanalysed all existing Chandra data of Abell 2744
gence level was calculated. (listed in Owers et al. (2011)), using CIAO 4.3 with the cali-
c 0000 RAS, MNRAS 000, 000–000
9. The galaxy cluster merger Abell 2744 9
bration database CALDB 4.4.2. We cleaned the data using front in the detached nearby NW ‘interloper’, which they
the standard procedure3 and kept events with grades 0, 2, guessed was falling into the main cluster. Owers et al. (2011)
3, 4 and 6. We removed the ACIS particle background as obtained a further 101 ksec of ACIS-I imaging. The deeper
prescribed for ‘VFAINT’ mode, and applied gain map cor- data revealed the ‘Southern minor remnant core’ (SMRC)
rection, together with Pulse Hight Amplitude (PHA) and to be colder (TX ∼ 7.5 keV) than its surroundings, with a
pixel randomisation. Point sources were identified and re- high temperature region to the SE (TX > 15 keV) that they
moved, and the sky background to be subtracted from spec- interpreted as a shock front. They concluded the SMRC had
tral fits was generated from Blank-Sky observations using been a low-mass bullet that has passed through the ‘North-
the acis bkgrnd lookup script. We fit spectra using XSPEC ern major remnant core’ (NMRC), leaving central tidal de-
V12.6.0q (Arnaud 1996) and we adopt Vapec thermal emis- bris (CTD). The pressure ratio of ∼ 3:1 across the shock
sion model from atomic data in the companion Astrophys- front corresponds to a sky-projected shock velocity of 2150
ical Plasma Emission Database (Smith et al. 2001), allow- km/s (for an average temperature TX ∼ 8.6 keV). Owers
ing for the variation of several individual elemental abun- et al. (2011) reversed the Kempner & David (2004) model
dances during the spectral fittings. Galactic photoelectric of the interloper, concluding that it came originally from the
absorption is incorporated using the wabs model (Morrison South, has already passed through the main body of the host
& McCammon 1983). Spectral channels are grouped to have cluster with a large impact parameter, and is now climbing
at least 20 counts/channel. Energy ranges are restricted to out towards the N-NW.
0.5–9.5 keV. Metal abundances are quoted relative to the Owers et al. (2011) also obtained spectra of more than
solar photospheric values of Anders & Grevesse (1989), and 1200 galaxies with the Anglo-Australian Telescope multi-
the spectral fitting parameter errors are 1-σ unless stated fibre AAOmega spectrograph, and confirmed the velocity
otherwise. bi-modality of the cluster galaxies. One component of galax-
There is a known reduction of quantum efficiency at en- ies near the SMRC has a peculiar velocity of 2300 km/s; a
ergies below 1 keV due to a build-up of molecular contami- separate component near the NMRC has a peculiar veloc-
nants on the optical blocking filter (or on the CCD chips)4 . ˜
ity of about −1600km/s and enhanced metal abundances
To prevent this from affecting our measurement of low en- (∼ 0.5 Solar). Assuming that the Northern and Southern
ergy line abundances, hydrogen column density and overall cores have the same velocities as the ‘apparently’ associated
gas temperature, we fix the column density to the cluster’s galaxy populations, they de-projected the velocity of this
nominal value of 1.6×1020 cm−2 (Dickey & Lockman 1990). collision to Mach 3.31 or nearly 5000 km/s.
To be conservative, we present results from only those re- An interesting prediction of this scenario is that the in-
gions of the CCDs best-suited for velocity analysis (ACIS-I tracluster gas should show a strong radial velocity gradient
pointings 8477 and 8557); for an independent sanity check, of ∼ 4000 km/s or ∆ z ∼ 0.014 from North to South. Since
we also repeat the analysis with ACIS-S pointing 2212. We the intracluster medium is enriched with heavy elements, ra-
exclude pointings 7712 and 7915 because the regions of in- dial velocity measurements could be carried out directly by
terest cross multiple CCDs, and interchip gain fluctuations measuring the Doppler shift of emission lines in the X-ray
could introduce spurious systematic effects when measuring spectrum (Dupke & Bregman 2001a,b; Andersson & Made-
the redshifts from X-ray spectra. jski 2004; Dupke et al. 2007) or through changes in line
Since spectral models are (weakly) degenerate with clus- broadening due to turbulence (Inogamov & Sunyaev 2003;
ter redshift, it is common for simultaneous fitting routines Sunyaev et al. 2003; Pawl et al. 2005). The former requires
to get stuck in local χ2 minima before they reach the global high photon counts within the spectral lines and excellent
best fit. We circumvent this via an iterative approach. We control of instrumental gain but could, in principle, be mea-
perform initial fits while varying the redshift values within sured with current X-ray spectrometers. The latter requires
reasonable ranges (via the command STEPPAR in XSPEC). very high spectral resolution that should become available
We then fix the redshift, and refit full spectral models to in- through the future ASTRO-H and IXO satellites (however,
fer gas temperature, metal abundances and normalisations. see Sanders et al. 2010, 2011).
Subsequently, we use these best-fit values as inputs in a new The existing data are not ideal for measurements of
fit with the redshift free to vary. This provides a more re- ICM velocity structure with high precision due to the varia-
liable estimation of the error on the redshift measurement. tion of gain expected from the analysis of multiple observa-
We repeat the process until the best-fit redshift no longer tions with different pointings and in different epochs. How-
changes between iterations. ever, the expected radial velocity gradient of 4000km/s, pre-
dicted from the optical work of Owers et al. (2011) is higher
than the expected interchip and intrachip gain variations,
3.2 Previous X-ray observations & interpretations so that it becomes possible to test, even if just for consis-
tency, the proposed merger configuration, as we did in this
Abell 2744 shows an extremely disturbed X-ray morphology
work. Chandra has a good gain temporal stability (Grant
(compare Fig. 5). With 25 ksec of Chandra ACIS-S3 imag-
2001) and we shall control for spatial variations by perform-
ing, Kempner & David (2004) decided it is the aftermath
ing resolved spectroscopy of multiple cluster regions using
of a N-S collision between similar mass proto-clusters, at
the same CCD location in different ACIS-I pointings.
Mach > 2.6. They also found tentative evidence for a cold
3.3 Velocity measurements
3 http://cxc.harvard.edu/ciao/guides/acis data.html
4 http://cxc.harvard.edu/cal/Links/Acis/ We perform our velocity analysis twice: for the regions of
acis/Cal prods/qeDeg/index.html interest defined by Owers et al. (2011), then for the sub-
c 0000 RAS, MNRAS 000, 000–000
10. 10 J. Merten et al.
structures prominent in our lensing mass maps. These are
respectively indicated by green or blue circles in Fig. 5.
We find temperature and metal abundance values for
the NMRC, CTD and SMRC regions of 8.29±0.67 keV and
0.55±0.18 Solar (χν =1.06 for 234 degrees of freedom);
9.51±0.47 keV and 0.27±0.08 Solar (χν =1.03 for 469 de-
grees of freedom); and 7.98±0.76 keV and 1.17±0.51 Solar
(χν =1.01 for 143 degrees of freedom). These values are
derived from simultaneous fits to pointings 8477, 8557 and
2212. The values are consistent with those obtained through
individual spectral fits for each instrument/observation (two
independent ACIS-I and one ACIS-S). Our temperature and
abundance measurements are thus consistent with the val-
ues found by Owers et al. (2011). Given that the observa-
tions were not tailored for velocity measurements (i.e. we
cannot exclude temporal or inter-chip gain variations), we
conservatively include in our velocity error bars secondary
χ2 minima that are separated from the global minimum at Figure 5. X-ray image of Abell 2744, overlaid with our lensing
mass reconstruction (magenta). The velocity gradient is maximal
less than 90% confidence in the velocity measurements.
between three regions of interest named by Owers et al. (2011)
Our velocity measurements are consistent with the pres- and circled in green (Southern minor remnant core, Northern ma-
ence of a gradient between the NMRC and SMRC, but in jor remnant core, and Central tidal debris). A velocity gradient
the opposite sense to that expected from Owers et al. (2011) is also detected between the regions of interest from our lensing
interpretation. Using the ACIS-S, the CCD with the best mass reconstruction, circled in blue (Core and NW) as well as the
spectral response, the velocity difference between NMRC interloper.
and SMRC is found to be > 1500 km/s at 90% confidence.
The observations (8477 and 8557) indicate a higher
magnitude for the velocity gradient detected. However, the
central values of the best-fit redshifts in each observation
are found to be significantly discrepant, introducing larger
uncertainties. The reason for this discrepancy is not clear,
but it is possibly related to the observed focal plane tem-
perature variation of about 0.9C between these observations
(Catherine Grant Personal Communication). This explana-
tion is supported by the marginally significant observed sys-
tematic difference of ∼ 10%-20% in the best-fit values of
temperatures and abundances measured for the same re-
gions in these different ACIS-I observations. This effect was
also noted in Owers et al. (2011) (Owers, M. Personal Com-
munication). Therefore, although our analysis is consistent
with a high velocity gradient, the direction of the gradient
does not corroborate the merger configuration suggested by
Owers et al. (2011).
Having the lensing mass reconstruction shown in the
previous sections at hand we can study the line-of-sight
gas velocity mapping for the regions of interest (Core and
NW) more precisely. Using all three exposures, we mea-
sure their gas temperatures and metal abundances to be Figure 6. Simultaneous fits to the redshifts of the Core re-
TXCore =10.50±0.57 keV, ACore =0.28±0.11 Solar with a gion and the NW substructure using two ACIS-I and one ACIS-S
reduced chi-squared χν =0.90 for 466 degrees of freedom pointings. The two contours correspond to 68% (inner) and 90%
and TXN W =10.22±0.54 keV, AN W =0.40±0.09 Solar with (outer) confidence levels, and the diagonal line indicates equal
redshifts to guide the eye. The Core mass clump appears signifi-
χν =0.91 for 477 degrees of freedom.
cantly redshifted when compared to the NW mass clump.
Despite their similar gas properties, these regions show
a velocity gradient > 5200 km/s at near 90% confidence,
even including a conservative (1σ) intra-chip gain variation
of 1000 km/s (e.g. Dupke & Bregman 2006) per CCD in
the error budget, in quadrature. A contour plot of the ve-
locity difference is shown in Fig. 6. This result is consistent the ACIS-I observations shows the same velocity trend, but
with the idea that the Southern Core mass is redshifted with with a reduction in the absolute velocity difference (an up-
respect to other structures. Nonetheless, these calculations per limit of <4400 km/s at the 90% confidence level). Deeper
should be taken with caution due to the uncertainties related Chandra observations, specifically tailored for velocity mea-
to temporal gain variations and inter-chip gain variations surements will be crucial to further reduce uncertainties and
between ACIS-I and ACIS-S. The same analysis using only to better constrain the velocity structure of this cluster.
c 0000 RAS, MNRAS 000, 000–000
11. The galaxy cluster merger Abell 2744 11
4 INTERPRETING THE MERGER tained from dark matter stripping in Abell 3827 (Williams
& Saha 2011; Carrasco et al. 2010) and the ellipticities of
Abell 2744 is undoubtedly undergoing a complicated merg-
dark matter halos (Miralda-Escud´ 2002), with implications
e
ing process on a large cluster scale. Progressively more
discussed in Feng et al. (2010).
detailed studies, culminating in our lensing reconstruction
(Sec. 2) and X-ray analysis (Sec. 3), have only agreed that
the merger is more complex than previously thought. For
4.2 Northern, the bullet
example, the likely explanation for the gas velocity gradient
in the opposite direction to that expected by Owers et al. Our analysis of the Northern mass substructure (upper-left
(2011) is that the NMRC is not the main cluster. Our lens- quadrant of Fig. 1) confirms the overall North-South merg-
ing mass reconstruction shows that the deepest gravitational ing scenario proposed by several authors in the past. We
potential is by far the Southern ‘Core’ structure, which is find a mass ratio of ∼ 2.6 between the Core and the North-
roughly coincident with Owers et al.’s SMRC but slightly ern clump, roughly supporting the 3:1 merging scenario of
further offset from the Compact Core. We also find three Boschin et al. (2006), but we identify the Northern sub-
separate mass concentrations to the North, Northwest and clump as the less massive progenitor. This conclusion is ro-
West. Overlaying the lensing mass reconstruction and X-ray bust, with no strong lensing features revealed by even our
emission in Fig. 1 reveals a complex picture of separations high resolution HST imaging in the Northern structure, as
between the dark matter and baryonic components. To in- would have been expected for the reversed mass ordering
terpret the sequence of events that led up to this present proposed by Owers et al. (2011).
state, we shall now tour the regions of interest, with more X-ray emission in the Northern mass substructure lags
detailed discussions. behind the dark matter as expected. We measure a separa-
tion of ∼ 30 to the South. This is a similar separation to
that in the core but, due to the lower surface-mass density
4.1 Core, the massive clump in this region, constraints on the collisional cross-section are
less significant.
According to our lensing analysis, the Core region (lower-
left quadrant of Fig. 1) is by far the most massive structure
within the merging system (c.f. Tab. 2). All the strong lens-
4.3 Northwestern, the ghostly and dark clumps
ing features can be seen within this clump. We find no large
separation between the distribution of mass and baryonic By far the most interesting structure is located to the North-
components. The mass peak is centred amongst the bright west of the cluster field (upper-right quadrant of Fig. 1). Our
cluster member galaxies (within 1σ errors, it is consistent lensing analysis (Sec. 2.5) shows it to be the second-most
with the position of the BCG) and only 22 ± 12 arcseconds massive structure. A separate region of X-ray emission also
from a peak of X-ray emission identified by Owers et al. lies to the Northwest, called the NW interloper by Owers
(2011). We support the general conclusion of Owers et al. et al. (2011). Furthermore, in 54% of our weak lensing boot-
(2011) that the major-merger in Abell 2744 is similar to that strap realisation we identified a second peak in the more
of the Bullet Cluster as it would be seen at a large inclina- Western area of the NW mass clump (see Fig. 1), rendering
tion with respect to the plane of the sky. However, we reverse it difficult to say if we indeed see a single, separate dark
the ordering of the major and minor mass components. matter structure and to derive decisive separation between
One can infer constraints on the collisional cross-section dark matter, X-ray luminous gas and bright cluster mem-
of dark matter from the separation between peaks in the ber galaxies. However, the separation between our different
lensing and X-ray maps. For the Bullet Cluster, Markevitch possible NW mass peak positions and the NW interloper is
et al. (2004) found σ/m < 5 cm2 g−1 , and for the Baby Bul- large (> 150 kpc for NW2). When compared to the more
let, Bradaˇ et al. (2008) found σ/m < 4 cm2 g−1 . In the Core
c prominent Eastern mass peak NW1, which is identified in
of Abell 2744, we observe a projected 17 separation that, if 95% of the bootstrap realisations, the distance is even at
the inclination is ∼ 30◦ away from the line-of-sight (Owers least 400 kpc.
et al. 2011), is a physical separation similar to that in the With a de-projected temperature of ∼ 5 keV (Owers
other bullet clusters. For an order of magnitude analysis, et al. 2011) the NW interloper should have r500 ∼ 1.34 Mpc,
we measure the mean surface-mass density within 150 kpc (Evrard et al. 1996) and M500 ∼ 4 − 5 × 1014 M (e.g. Fig. 8
of the mass peak Σ 0.30+0.08 g cm−2 , so that the scat-
−0.07 of Khosroshahi et al. (2007)). Its 0.1 − 10.0keV unabsorbed
tering depth τs = σ/m Σ. With the assumption τs < 1, luminosity is 2.5×1044 erg/s, consistent with its gas temper-
which is justified due to the observed dark matter-gas sep- ature of 4 − 5 keV (Khosroshahi et al. 2007; D´ ıaz-Gim´nez
e
aration, we deduce σ/m < 3 ± 1 cm2 g−1 . This system may et al. 2011). Assuming a β (from a King-like profile) of 0.67,
therefore yield one of the tightest constraints on the interac- typical for clusters, the clump would have ∼ 0.95 × 1014 M
tion cross-section of dark matter, based on such analysis. A within 250 kpc, similar to the N clump, and should have
full numerical simulation to interpret the cluster configura- been easily detected in the lensing analysis. The interloper
tion would be ideal, especially given the uncertainty in the thus appears to be an X-ray feature with no associated dark
collision angle with respect to the plane of the sky. Indeed, matter or galaxies, and we therefore dub it the ‘ghost’ clus-
even tighter constraints (σ/m < 0.7 cm2 g−1 ) were obtained ter.
from the Bullet Cluster by (Randall et al. 2008), who inter- There is also a clear separation between the peak of
preted the offsets between all three cluster components via the NW mass clump and any cluster member galaxies, so
tailored hydrodynamical simulations. Additional constraints we call this the ‘dark’ cluster. Contours of the lensing mass
on the collisional cross-section have also recently been ob- reconstruction extend towards the West, where indeed we
c 0000 RAS, MNRAS 000, 000–000
12. 12 J. Merten et al.
find a pair of giant ellipticals (see Fig. 1). However, with the
limited resolution of the VLT weak lensing reconstruction, it
is impossible to tell whether this is a binary mass structure,
though there are clear indications for that in our analysis of
this mass clump.
The separation between all three mass components
makes this a real puzzle and it should be stressed that such
a peculiar configuration is observed for the first time. It
may pose another serious challenge for cosmological models
of structure formation. One possible explanation was sug-
gested by Owers et al. (2011), who describe it as a ram-
pressure slingshot (e.g. Markevitch & Vikhlinin 2007). In
this interpretation, after first core-passage, gas initially trails
its associated dark matter but, while the dark matter slows
down, the gas slingshots past it due to a combination of low
ram-pressure stripping and adiabatic expansion and cooling,
which enhances the cold front temperature contrast (Bialek
et al. 2002). There is indeed a clear velocity gradient be-
tween the NW interloper and the main cluster core (see
Figure 7. Chandra ACIS-S3 X-ray image of Abell 2744,
Sec. 3). Such effects have also been observed e.g. in Abell 168
smoothed with a 3 pixel kernel Gaussian. We analyse the cluster’s
(Hallman & Markevitch 2004), in a joint weak-lensing and X-ray profile in three rectangular regions, each 150 in length and
X-ray study of Abell 754 (Okabe & Umetsu 2008) and in nu- radiating from the cluster centre (marked by the blue circle). We
merical simulations (Ascasibar & Markevitch 2006; Mathis find marginally significant excess X-ray emission in the central
et al. 2005), although at a much smaller separations be- rectangular region, which extends towards the Western clump
tween dark matter and gas. The more than 100 separation (marked by the green circle).
in Abell 2744 suggests either that the slingshot scenario is
unlikely or that some amplifying mechanism is in place. We
shall return to this issue, proposing an interpretation of the the core towards the Western clump (along the central box)
entire cluster merger, at the end of this section. is shown in Fig. 8. The slope of the profile changes at a
radius ∼ 135 = 612 kpc away from the centre, becoming
significantly shallower. If we take that point of transition as
4.4 Western, the stripped clump the location of the remaining gas core, we obtain a separa-
The Western substructure (lower-right quadrant of Fig. 1) tion between the gas and dark matter of 30 . Similar results
has not yet been discussed in the literature, but several clus- are found with the shallower observation 7915 using ACIS-I.
ter member galaxies are found in this area. We find a promi- We find a slight offset between the peak of the lens-
nent weak gravitational lensing signal of ∼ 1.0 × 1014 M ing mass reconstruction and the most luminous nearby clus-
within 250 kpc. This should correspond to an X-ray bright- ter member galaxies. However, there is large uncertainty in
ness higher than the NW interloper, for the same gas tem- the position of the lensing peak because this lies outside
perature. However, we detect no X-ray emission. The best the HST imaging area. Our weak lensing analysis uses only
X-ray data (ACIS-I pointings 8477 & 8557) do not cover the VLT imaging, and there are no strong lensing constraints, so
region of this lensing signal, but the ACIS-S pointing 2212 the mass reconstruction has a broad central plateau. Addi-
indicates no excess diffuse gas above the cosmic background tional HST observations would provide an ideal foundation
and the extended tail of the cluster’s outskirts. To match to better understand the Western area, which turns out to
these observations, the Western clump must have been com- be playing a significant role in the overall merger.
pletely stripped of its ICM, so we dub it the ‘stripped’ clump.
The only slight excess X-ray emission nearby is a faint
4.5 One possible interpretation
extension of the main cluster core towards the West (‘ridge
c’ in Owers et al. 2011). This roughly links the Western We shall now try to develop a possible explanation of the
clump to the Northern clump, and is consistent with rem- complex merging scenario that has taken place in Abell 2744.
nants stripped by ram-pressure during a secondary merging To recap, we find four mass clumps (Core, N, NW, W) with
event (NE–SW), almost perpendicular to the main merg- approximate masses 2.2, 0.8, 1.1, 1.1×1014 M , respectively.
ing event projected in the sky plane. To quantify the excess The Core, N and W clumps are relatively close to BCGs and
X-ray emission, we measure counts inside three equal-size, hot gas. The NW structure, on the other hand, contains
non-overlapping regions extending from the cluster core to separated dark matter, gas and galaxies.
a radial distance of 150 ≈ 680 kpc, as shown in Fig. 7. We propose that the current configuration is the result
To prepare the data for this analysis, we divide the image of a near simultaneous double merger, as illustrated in Fig. 9.
by the exposure map following the standard procedure, re- The first merger, in the NE-SW direction, had a character-
move hot pixels and remove point sources using the dmfilth istic path of 208 (plane of the sky distance between N and
routine in CIAO. We find a marginal excess in the central W clumps) or ∼ 0.95 Mpc (assuming no line of sight veloc-
box (1030±32 counts), which points to the Western clump, ity component). The Western clump probably passed closest
above the Northern (965±31 counts) and Southern (876±29 through the main cluster, as it had its ICM ram-pressure
counts) boxes. The cluster’s surface brightness profile from stripped completely. The second merger, in the SE-NW di-
c 0000 RAS, MNRAS 000, 000–000
13. The galaxy cluster merger Abell 2744 13
rection, had a characteristic path of 117 / sin(27◦ ) ≈ 1.17
Mpc, if we assume the inclination of that merger sug-
gested by Owers et al. (2011) of 27◦ . The mergers happened
around 0.12–0.15 Gyr ago, with a characteristic velocity of
∼ 4000 km/s as indicated by the galaxy velocity difference
and the ICM gas velocity measurements.
It is possible that the merger in the SE-NW direction
could even have consisted of three initial substructures: the
Core and two consecutive clumps (with a combined mass
(within 250 kpc of each core) of ∼ 1.2 × 1014 M ) falling
along a filament. Those smaller clumps would be acceler-
ated by the gravitational pull of the main cluster (plus the
Northern and Western clumps, which were merging per-
pendicularly). The ram-pressure slingshot in these clumps
could be enhanced by a combination of an initially stronger
gravitational field and perhaps a posterior reduction in ram-
pressure due to the ‘puff-up’ of the gas density due to the
recent merger of Core+N+W, similar to the density config-
uration of the main component in the Bullet Cluster. The
Figure 8. Surface brightness profile (in arbitrary units) of the
central rectangular region shown in Fig. 7. The left hand side combined effect would throw the gas component ahead of its
starts at the X-ray centre, and the arrow denotes the approxi- associated dark matter, forming the ‘ghost’ cluster, which is
mate position of the Western mass clump. The abrupt change in now the interloper. The two dark matter clumps left behind
profile slope at roughly the same location is highlighted by the would now form the possibly double-centred ‘dark’ clump.
intersection with the horizontal line. This unusual scenario fits the current observations. It
explains the clear extension of the NW clump towards the
West and provides probably a mechanism to create such a
huge separation between dark matter and gas in the NW
area. However, the double peak in the NW clump needs to
be confirmed by an additional HST pointing to cover its full
area. The discovery of a second distinct mass peak in the
NW clump would support our merger scenario and also de-
crease the large measured separations between dark matter,
gas and bright cluster galaxies. Also the ghostly interloper
has not been covered by the current HST observations and
the Western clump falls between chip gaps in the longer
Chandra exposures, so further observations will still be re-
quired. Our suggestions will also eventually require verifica-
tion via a set of well-tailored numerical simulations and have
to be taken with some caution at the current stage. Cosmo-
logical boxes have to be explored (compare Meneghetti et al.
2010, 2011) in order to find similar configurations and de-
tailed, hydrodynamical simulations need to repeat and con-
firm our findings.
5 CONCLUSIONS
We present a detailed strong lensing, weak lensing and X-ray
analysis of the merging cluster of galaxies Abell 2744. Earlier
studies (Kempner & David 2004; Boschin et al. 2006; Braglia
Figure 9. Our proposed merging scenario, illustrated in time-
ordered sequence, to explain the current configuration (panel 4).
et al. 2009; Owers et al. 2011) concluded that Abell 2744 is
We suggest that Abell 2744 is the result of a nearly simultaneous undergoing a complicated merging event. We find that it
double merger: one in the NE-SW direction and another in the is even more complex than previously thought, unleashing
NW-SE direction, which may even have consisted of three sepa- a variety of exciting effects. We dub this merger therefore
rate structures falling along a filament. Blue colour shall indicate Pandora’s cluster.
the innermost dark matter cores of the clumps, where else their Deep, three-band HST-imaging reveals a variety of
respective ICM is shown in red. strong-lensing features in the core of the cluster. From our
comprehensive strong-lensing modelling of the central mass
distribution, we identify a total number of not less than 34
multiple images, in 11 multiple-image systems, together with
their respective redshifts. The strong lens systems are listed
in Tab. 1 and the finely resolved critical curve of the cluster
c 0000 RAS, MNRAS 000, 000–000