1) The galaxy merger rate over cosmic time is a fundamental measure of galaxy evolution. Mergers are thought to drive galaxy assembly, rapid star formation, and the formation of spheroidal galaxies and supermassive black holes.
2) While simulations predict that the dark matter halo merger rate increases strongly with redshift, the theoretical predictions for the galaxy merger rate remain uncertain due to the complex relationship between galaxies and dark matter halos.
3) The paper aims to address this problem by calibrating three observational galaxy merger rate estimators - close galaxy pairs, G-M20 morphology, and asymmetry - using hydrodynamic merger simulations. This allows them to determine merger timescales and consistently apply them to observed merger fractions up to
DISCOVERY OF A GALAXY CLUSTER WITH A VIOLENTLY STARBURSTING CORE AT z = 2:506Sérgio Sacani
We report the discovery of a remarkable concentration of massive galaxies with extended X-ray
emission at zspec = 2:506, which contains 11 massive (M & 1011M) galaxies in the central 80kpc
region (11.6 overdensity). We have spectroscopically conrmed 17 member galaxies with 11 from CO
and the remaining ones from H. The X-ray luminosity, stellar mass content and velocity dispersion
all point to a collapsed, cluster-sized dark matter halo with mass M200c = 1013:90:2M, making it
the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this
structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive
galaxies classied as quiescent. The star formation rate (SFR) in the 80kpc core reaches 3400 M
yr 1 with a gas depletion time of 200 Myr, suggesting that we caught this cluster in rapid build-up
of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst
fraction ( 25%, compared to 3%-5% in the eld). The presence of both a collapsed, cluster-sized
halo and a predominant population of massive SFGs suggests that this structure could represent an
important transition phase between protoclusters and mature clusters. It provides evidence that the
main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster,
providing new insights into massive cluster formation at early epochs. The large integrated stellar
mass at such high redshift challenges our understanding of massive cluster formation.
The discovery of a giant arc associated with the galaxy cluster IDCS J1426.5+3508 at z=1.75 is reported. HST imaging reveals an arc-like object with a length-to-width ratio greater than 10, located near the brightest cluster galaxy. Attempts to measure the redshift of the arc through spectroscopy were unsuccessful. The existence of such a massive, strongly lensing cluster at this early epoch poses challenges for the standard ΛCDM model of structure formation.
The habitability of Proxima Centauri b - I. Irradiation, rotation and volatil...Sérgio Sacani
Proxima b is a planet with a minimum mass of 1.3 M⊕ orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass,
active star and the Sun’s closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b
and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet
and show that the planet currently receives 30 times more EUV radiation than Earth and 250 times more X-rays. We compute the time
evolution of the star’s spectrum, which is essential for modeling the flux received over Proxima b’s lifetime. We also show that Proxima
b’s obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet’s eccentricity and
level of triaxiality. Next we consider the evolution of Proxima b’s water inventory. We use our spectral energy distribution to compute
the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find
that Proxima b is likely to have lost less than an Earth ocean’s worth of hydrogen (EOH) before it reached the HZ 100–200 Myr after
its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We
conclude that Proxima b is a viable candidate habitable planet.
The birth of_a_galaxy_propelling_reionisation_with_the_faintest_galaxiesSérgio Sacani
This document summarizes a study that uses cosmological radiation hydrodynamics simulations to model dwarf galaxy formation and properties at z > 7. The study finds that metal-enriched star formation can occur in halos down to masses of ~106 M, especially in neutral regions, and that these smallest galaxies provide nearly 30% of the ionizing photon budget needed for reionization despite only hosting up to 104 M of stars. The study also finds that the galaxy luminosity function flattens above MUV = -12 at z = 10 and that the fraction of ionizing radiation escaping into the intergalactic medium decreases from 50% to 5% over the halo mass range of log M/M⊙ = 7.0-8.
A population of faint low surface brightness galaxies in the Perseus cluster ...Sérgio Sacani
We present the detection of 89 low surface brightness (LSB), and thus low stellar
density galaxy candidates in the Perseus cluster core, of the kind named ‘ultra-diffuse
galaxies’, with mean effective V-band surface brightnesses 24.8–27.1 mag arcsec−2
, total
V-band magnitudes −11.8 to −15.5 mag, and half-light radii 0.7–4.1 kpc. The candidates
have been identified in a deep mosaic covering 0.3 deg2
, based on wide-field
imaging data obtained with the William Herschel Telescope. We find that the LSB
galaxy population is depleted in the cluster centre and only very few LSB candidates
have half-light radii larger than 3 kpc. This appears consistent with an estimate of
their tidal radius, which does not reach beyond the stellar extent even if we assume
a high dark matter content (M/L = 100). In fact, three of our candidates seem to
be associated with tidal streams, which points to their current disruption. Given that
published data on faint LSB candidates in the Coma cluster – with its comparable central
density to Perseus – show the same dearth of large objects in the core region, we
conclude that these cannot survive the strong tides in the centres of massive clusters.
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.
Extensive Noachian fluvial systems in Arabia Terra: Implications for early Ma...Sérgio Sacani
Valley networks are some of the strongest lines of evidence for
extensive fluvial activity on early (Noachian; >3.7 Ga) Mars. However,
their purported absence on certain ancient terrains, such as
Arabia Terra, is at variance with patterns of precipitation as predicted
by “warm and wet” climate models. This disagreement has contributed
to the development of an alternative “icy highlands” scenario,
whereby valley networks were formed by the melting of highland ice
sheets. Here, we show through regional mapping that Arabia Terra
shows evidence for extensive networks of sinuous ridges. We interpret
these ridge features as inverted fluvial channels that formed in
the Noachian, before being subject to burial and exhumation. The
inverted channels developed on extensive aggrading flood plains. As
the inverted channels are both sourced in, and traverse across, Arabia
Terra, their formation is inconsistent with discrete, localized sources
of water, such as meltwater from highland ice sheets. Our results are
instead more consistent with an early Mars that supported widespread
precipitation and runoff.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
The document summarizes a study of an extreme molecular cloud in the Antennae galaxies that has properties consistent with forming a globular cluster. ALMA observations reveal a cloud with a radius of 24 pc and mass greater than 5 million solar masses. While capable of forming a globular cluster, a lack of associated thermal radio emission indicates star formation has not yet begun to alter the environment, suggesting the cloud is in an early stage of evolution. For the cloud to be confined as observed, an external pressure over 10,000 times greater than typical interstellar pressure is required, supporting the theory that high pressures are needed to form globular clusters in extreme environments like mergers.
DISCOVERY OF A GALAXY CLUSTER WITH A VIOLENTLY STARBURSTING CORE AT z = 2:506Sérgio Sacani
We report the discovery of a remarkable concentration of massive galaxies with extended X-ray
emission at zspec = 2:506, which contains 11 massive (M & 1011M) galaxies in the central 80kpc
region (11.6 overdensity). We have spectroscopically conrmed 17 member galaxies with 11 from CO
and the remaining ones from H. The X-ray luminosity, stellar mass content and velocity dispersion
all point to a collapsed, cluster-sized dark matter halo with mass M200c = 1013:90:2M, making it
the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this
structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive
galaxies classied as quiescent. The star formation rate (SFR) in the 80kpc core reaches 3400 M
yr 1 with a gas depletion time of 200 Myr, suggesting that we caught this cluster in rapid build-up
of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst
fraction ( 25%, compared to 3%-5% in the eld). The presence of both a collapsed, cluster-sized
halo and a predominant population of massive SFGs suggests that this structure could represent an
important transition phase between protoclusters and mature clusters. It provides evidence that the
main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster,
providing new insights into massive cluster formation at early epochs. The large integrated stellar
mass at such high redshift challenges our understanding of massive cluster formation.
The discovery of a giant arc associated with the galaxy cluster IDCS J1426.5+3508 at z=1.75 is reported. HST imaging reveals an arc-like object with a length-to-width ratio greater than 10, located near the brightest cluster galaxy. Attempts to measure the redshift of the arc through spectroscopy were unsuccessful. The existence of such a massive, strongly lensing cluster at this early epoch poses challenges for the standard ΛCDM model of structure formation.
The habitability of Proxima Centauri b - I. Irradiation, rotation and volatil...Sérgio Sacani
Proxima b is a planet with a minimum mass of 1.3 M⊕ orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass,
active star and the Sun’s closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b
and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet
and show that the planet currently receives 30 times more EUV radiation than Earth and 250 times more X-rays. We compute the time
evolution of the star’s spectrum, which is essential for modeling the flux received over Proxima b’s lifetime. We also show that Proxima
b’s obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet’s eccentricity and
level of triaxiality. Next we consider the evolution of Proxima b’s water inventory. We use our spectral energy distribution to compute
the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find
that Proxima b is likely to have lost less than an Earth ocean’s worth of hydrogen (EOH) before it reached the HZ 100–200 Myr after
its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We
conclude that Proxima b is a viable candidate habitable planet.
The birth of_a_galaxy_propelling_reionisation_with_the_faintest_galaxiesSérgio Sacani
This document summarizes a study that uses cosmological radiation hydrodynamics simulations to model dwarf galaxy formation and properties at z > 7. The study finds that metal-enriched star formation can occur in halos down to masses of ~106 M, especially in neutral regions, and that these smallest galaxies provide nearly 30% of the ionizing photon budget needed for reionization despite only hosting up to 104 M of stars. The study also finds that the galaxy luminosity function flattens above MUV = -12 at z = 10 and that the fraction of ionizing radiation escaping into the intergalactic medium decreases from 50% to 5% over the halo mass range of log M/M⊙ = 7.0-8.
A population of faint low surface brightness galaxies in the Perseus cluster ...Sérgio Sacani
We present the detection of 89 low surface brightness (LSB), and thus low stellar
density galaxy candidates in the Perseus cluster core, of the kind named ‘ultra-diffuse
galaxies’, with mean effective V-band surface brightnesses 24.8–27.1 mag arcsec−2
, total
V-band magnitudes −11.8 to −15.5 mag, and half-light radii 0.7–4.1 kpc. The candidates
have been identified in a deep mosaic covering 0.3 deg2
, based on wide-field
imaging data obtained with the William Herschel Telescope. We find that the LSB
galaxy population is depleted in the cluster centre and only very few LSB candidates
have half-light radii larger than 3 kpc. This appears consistent with an estimate of
their tidal radius, which does not reach beyond the stellar extent even if we assume
a high dark matter content (M/L = 100). In fact, three of our candidates seem to
be associated with tidal streams, which points to their current disruption. Given that
published data on faint LSB candidates in the Coma cluster – with its comparable central
density to Perseus – show the same dearth of large objects in the core region, we
conclude that these cannot survive the strong tides in the centres of massive clusters.
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.
Extensive Noachian fluvial systems in Arabia Terra: Implications for early Ma...Sérgio Sacani
Valley networks are some of the strongest lines of evidence for
extensive fluvial activity on early (Noachian; >3.7 Ga) Mars. However,
their purported absence on certain ancient terrains, such as
Arabia Terra, is at variance with patterns of precipitation as predicted
by “warm and wet” climate models. This disagreement has contributed
to the development of an alternative “icy highlands” scenario,
whereby valley networks were formed by the melting of highland ice
sheets. Here, we show through regional mapping that Arabia Terra
shows evidence for extensive networks of sinuous ridges. We interpret
these ridge features as inverted fluvial channels that formed in
the Noachian, before being subject to burial and exhumation. The
inverted channels developed on extensive aggrading flood plains. As
the inverted channels are both sourced in, and traverse across, Arabia
Terra, their formation is inconsistent with discrete, localized sources
of water, such as meltwater from highland ice sheets. Our results are
instead more consistent with an early Mars that supported widespread
precipitation and runoff.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
The document summarizes a study of an extreme molecular cloud in the Antennae galaxies that has properties consistent with forming a globular cluster. ALMA observations reveal a cloud with a radius of 24 pc and mass greater than 5 million solar masses. While capable of forming a globular cluster, a lack of associated thermal radio emission indicates star formation has not yet begun to alter the environment, suggesting the cloud is in an early stage of evolution. For the cloud to be confined as observed, an external pressure over 10,000 times greater than typical interstellar pressure is required, supporting the theory that high pressures are needed to form globular clusters in extreme environments like mergers.
This document presents a sample of 151 dwarf galaxies that exhibit optical spectroscopic signatures of accreting massive black holes. The sample was identified by systematically searching ~25,000 emission-line galaxies from the Sloan Digital Sky Survey with stellar masses comparable to or less than the Large Magellanic Cloud. Many of the galaxies show narrow-line signatures of black hole accretion, and some also exhibit broad H-alpha emission, indicating gas orbiting in the deep potential of a massive black hole. This increases the number of known active galaxies in this low stellar mass range by over an order of magnitude. The median stellar mass of the host galaxies is around 108.5 solar masses, around 1-2 magnitudes fainter than previous samples of
Beyond the Kuiper Belt Edge: New High Perihelion Trans-Neptunian Objects With...Sérgio Sacani
We are conducting a survey for distant solar system objects beyond the Kuiper
Belt edge ( 50 AU) with new wide-field cameras on the Subaru and CTIO tele-
scopes. We are interested in the orbits of objects that are decoupled from the
giant planet region in order to understand the structure of the outer solar sys-
tem, including whether a massive planet exists beyond a few hundred AU as first
reported in Trujillo and Sheppard (2014). In addition to discovering extreme
trans-Neptunian objects detailed elsewhere, we have found several objects with
high perihelia (q > 40 AU) that differ from the extreme and inner Oort cloud
objects due to their moderate semi-major axes (50 < a < 100 AU) and eccen-
tricities (e . 0.3). Newly discovered objects 2014 FZ71 and 2015 FJ345 have
the third and fourth highest perihelia known after Sedna and 2012 VP113, yet
their orbits are not nearly as eccentric or distant. We found several of these high
perihelion but moderate orbit objects and observe that they are mostly near Nep-
tune mean motion resonances and have significant inclinations (i > 20 degrees).
These moderate objects likely obtained their unusual orbits through combined
interactions with Neptune’s mean motion resonances and the Kozai resonance,
similar to the origin scenarios for 2004 XR190. We also find the distant 2008
ST291 has likely been modified by the MMR+KR mechanism through the 6:1
Neptune resonance. We discuss these moderately eccentric, distant objects along
with some other interesting low inclination outer classical belt objects like 2012
FH84 discovered in our ongoing survey.
A giant ring_like_structure_at_078_z_086_displayed_by_gr_bsSérgio Sacani
This document describes the discovery of a giant ring-like structure in the observable universe displayed by 9 gamma ray bursts (GRBs) between redshifts of 0.78 and 0.86. The ring has a diameter of 1720 Mpc, over five times larger than the expected transition scale to homogeneity. The ring lies at a distance of 2770 Mpc with major and minor diameters of 43° and 30°, respectively. The probability of this structure occurring by random chance is calculated to be 2 × 10-6. This ring-shaped feature contradicts the cosmological principle of large-scale homogeneity and isotropy, and the physical mechanism responsible is unknown.
The kepler 10_planetary_system_revisited_by_harpsSérgio Sacani
This document discusses observations of the Kepler-10 planetary system made with the HARPS-N spectrograph. The observations resulted in improved precision for the masses of Kepler-10b and Kepler-10c. Kepler-10b's mass was determined to be 3.33 ± 0.49 Earth masses, confirming its rocky composition. Kepler-10c's mass of 17.2 ± 1.9 Earth masses makes it the first strong evidence of a class of more massive solid Neptune-sized planets with longer orbital periods. The improved mass measurements will help constrain models of the internal structure and composition of these planets.
Predictions of the_atmospheric_composition_of_gj_1132_bSérgio Sacani
GJ 1132 b is a nearby Earth-sized exoplanet transiting an M dwarf, and is amongst the most highly
characterizable small exoplanets currently known. In this paper we study the interaction of a magma
ocean with a water-rich atmosphere on GJ 1132b and determine that it must have begun with more
than 5 wt% initial water in order to still retain a water-based atmosphere. We also determine the
amount of O2
that can build up in the atmosphere as a result of hydrogen dissociation and loss.
We find that the magma ocean absorbs at most ∼ 10% of the O2 produced, whereas more than
90% is lost to space through hydrodynamic drag. The most common outcome for GJ 1132 b from our
simulations is a tenuous atmosphere dominated by O2
, although for very large initial water abundances
atmospheres with several thousands of bars of O2
are possible. A substantial steam envelope would
indicate either the existence of an earlier H2
envelope or low XUV flux over the system’s lifetime. A
steam atmosphere would also imply the continued existence of a magma ocean on GJ 1132 b. Further
modeling is needed to study the evolution of CO2
or N2
-rich atmospheres on GJ 1132 b.
This pilot survey used modest aperture telescopes to image 8 nearby spiral galaxies in order to search for stellar tidal streams. Ultra-deep imaging revealed 6 previously undetected extensive (up to 30 kpc) stellar structures likely from tidally disrupted satellites. A diversity of tidal feature morphologies was found, including great circle-like streams, remote shells, and jets emerging from disks. Simulations predict tidal debris should be common and match the observed variety, providing evidence minor mergers have shaped disk galaxies since z=1.
A super earth transiting a naked-eye starSérgio Sacani
This document summarizes the detection of transits of an exoplanet orbiting the star 55 Cnc, using photometry from the MOST space telescope. The transits match the period, phase, duration, and depth predicted for the innermost planet of 55 Cnc, designated 55 Cnc e. Analysis of the transit data indicates the planet has a mass of 8.57 Earth masses, a radius of 1.63 Earth radii, and a dense composition of rock and iron. This makes 55 Cnc e similar to other dense super-Earth exoplanets in short orbits, unlike lower-density super-Earths further from their stars. The brightness of 55 Cnc will enable further study of this transiting exoplanet system.
The green valley_is_a_red_herring_galaxy_zoo_reveals_two_evolutionary_pathwaysSérgio Sacani
This document summarizes research using data from Galaxy Zoo, SDSS, and GALEX to study how star formation is quenched in low-redshift galaxies. The key findings are:
1) Taking galaxy morphology into account, the "green valley" is not a single transitional state, as was previously thought.
2) Only a small population of blue early-type galaxies rapidly transition across the green valley as their morphology transforms from disk to spheroid and star formation is quenched quickly.
3) The majority of blue star-forming galaxies have significant disks and retain their late-type morphology as their star formation rates decline very slowly.
4) Different evolutionary pathways are observed for early- and late-type
Kinematics and simulations_of_the_stellar_stream_in_the_halo_of_the_umbrella_...Sérgio Sacani
This document summarizes a study of the stellar stream and substructures around the Umbrella Galaxy (NGC 4651). Deep imaging and spectroscopy were used to characterize the properties and kinematics of the stream. Tracer objects like globular clusters and planetary nebulae were identified and found to delineate a kinematically cold feature in position-velocity space. Dynamical modeling suggests the stream originated from the tidal disruption of a dwarf galaxy on a highly eccentric orbit about 6-10 billion years ago. This work demonstrates the feasibility of using discrete tracers to recover the kinematics and model the dynamics of low surface brightness stellar streams around distant galaxies.
The behaviour of_dark_matter_associated_with_4_bright_cluster_galaxies_in_the...Sérgio Sacani
The document presents new Hubble Space Telescope imaging and Very Large Telescope integral-field spectroscopy of galaxy cluster Abell 3827. It finds that each of the four central galaxies in the cluster's 10 kpc core retains an associated massive dark matter halo. At least one galaxy is offset from its dark matter halo, with an offset of 1.62+0.50−0.47 kpc, within the uncertainties of mass modeling and statistical errors. If interpreted as evidence for self-interacting dark matter, this offset implies a dark matter self-interaction cross-section of (1.7 ± 0.7)×10−4 cm2/g.
This document summarizes the discovery of a stellar tidal stream around the nearby dwarf galaxy NGC 4449 using deep imaging. The stream implies a massive dwarf spheroidal progenitor that is disrupting and depositing stars into NGC 4449's halo. The luminosity ratio between the two galaxies is around 1:50, but their dynamical mass ratio including dark matter may be 1:10-1:5. This "stealth merger" suggests that satellite accretion can significantly build up stellar halos of low-mass galaxies and possibly trigger their starbursts.
The fornax deep_survey_with_vst_i_the_extended_and_diffuse_stellar_halo_of_ng...Sérgio Sacani
We have started a new deep, multi-imaging survey of the Fornax cluster, dubbed Fornax Deep
Survey (FDS), at the VLT Survey Telescope. In this paper we present the deep photometry inside
two square degrees around the bright galaxy NGC 1399 in the core of the cluster. We found that
the core of the Fornax cluster is characterised by a very extended and diffuse envelope surrounding
the luminous galaxy NGC 1399: we map the surface brightness out to 33 arcmin (∼ 192 kpc)
from the galaxy center and down to μg ∼ 31 mag arcsec−2 in the g band. The deep photometry
allows us to detect a faint stellar bridge in the intracluster region on the west side of NGC 1399
and towards NGC 1387. By analyzing the integrated colors of this feature, we argue that it
could be due to the ongoing interaction between the two galaxies, where the outer envelope of
NGC 1387 on its east side is stripped away. By fitting the light profile, we found that exists a
physical break radius in the total light distribution at R = 10 arcmin (∼ 58 kpc) that sets the
transition region between the bright central galaxy and the outer exponential halo, and that the
stellar halo contributes for 60% of the total light of the galaxy (Sec. 3.5). We discuss the main
implications of this work on the build-up of the stellar halo at the center of the Fornax cluster.
By comparing with the numerical simulations of the stellar halo formation for the most massive
BCGs (i.e. 13 < logM200/M⊙ < 14), we find that the observed stellar halo mass fraction is
consistent with a halo formed through the multiple accretion of progenitors with stellar mass in
the range 108 − 1011 M⊙. This might suggest that the halo of NGC 1399 has also gone through
a major merging event. The absence of a significant number of luminous stellar streams and
tidal tails out to 192 kpc suggests that the epoch of this strong interaction goes back to an early
formation epoch. Therefore, differently from the Virgo cluster, the extended stellar halo around
NGC 1399 is characterised by a more diffuse and well-mixed component, including the ICL.
The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in th...GOASA
HR 5171 A exhibits a complex appearance based on AMBER/VLTI observations. The observations reveal an unusually large star of approximately 1315 solar radii at a distance of 3.6 kiloparsecs. The source is surrounded by an extended nebula. The observations also show a high level of asymmetry in the brightness distribution, which is attributed to the discovery of a companion star located in front of the primary. Analysis of visual photometry data indicates the system has an orbital period of 1304 days, providing evidence it is a contact or over-contact eclipsing binary. Modeling suggests a total system mass of 39-79 solar masses and a high mass ratio of at least 10 for the companion. The discovery of the
This document presents an analysis of transit spectroscopy observations of three exoplanets - WASP-12 b, WASP-17 b, and WASP-19 b - using the Wide Field Camera 3 instrument on the Hubble Space Telescope. The observations achieved almost photon-limited precision but uncertainties in the transit depths were increased by the uneven sampling of the light curves. The final transit spectra for all three planets are consistent with the presence of a water absorption feature at 1.4 microns, though the amplitude is smaller than expected from previous Spitzer observations possibly due to hazes. Due to degeneracies between models, the data cannot unambiguously constrain the atmospheric compositions without additional observations.
SPECTROSCOPIC CONFIRMATION OF THE EXISTENCE OF LARGE, DIFFUSE GALAXIES IN THE...Sérgio Sacani
We recently identified a population of low surface brightness objects in the field of the z = 0.023 Coma cluster,
using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these “ultradiffuse
galaxies” (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption
features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz=6280±120 km s−1 is within the 1σ velocity dispersion of the Coma cluster. The galaxy has an effective
radius of 4.3 ± 0.3 kpc and a Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ∼2re. We show that UDGs are located in a previously
sparsely populated region of the size—magnitude plane of quiescent stellar systems, as they are ∼6 mag fainter
than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent
galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are
needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same
luminosity or to those of galaxies with the same size.
The article discusses two gaseous regions discovered that have a chemical composition close to that of the early universe before the first stars formed. This finding demonstrates that metals dispersed unevenly throughout the universe, with implications for when the first generation of stars could have formed. The early universe began with hydrogen, helium, and trace amounts of lithium produced by Big Bang nucleosynthesis. Later, the first stars enriched and ionized the universe with heavier elements, though it appears this process was nonuniform based on the discovery of pockets of nearly pristine gas.
Ages and stratigraphy of lunar mare basalts a synthesisSérgio Sacani
This document summarizes ages and stratigraphy of lunar mare basalts based on a synthesis of crater counting efforts over more than 10 years. The key points are:
1) Lunar volcanism was active for almost 3 billion years, starting around 3.9-4 billion years ago and ceasing around 1.2 billion years ago.
2) Most basalts erupted during the late Imbrian period around 3.6-3.8 billion years ago.
3) Significantly fewer basalts were emplaced during the Eratosthenian period.
4) Basalts of possible Copernican age have only been found in limited areas of Oceanus Procellarum.
This document presents a sample of 151 dwarf galaxies that exhibit optical spectroscopic signatures of accreting massive black holes. The sample was identified by systematically searching ~25,000 emission-line galaxies from the Sloan Digital Sky Survey with stellar masses comparable to or less than the Large Magellanic Cloud. Many of the galaxies show narrow-line signatures of black hole accretion, and some also exhibit broad H-alpha emission, indicating gas orbiting in the deep potential of a massive black hole. This increases the number of known active galaxies in this low stellar mass range by over an order of magnitude. The median stellar mass of the host galaxies is around 108.5 solar masses, around 1-2 magnitudes fainter than previous samples of
Beyond the Kuiper Belt Edge: New High Perihelion Trans-Neptunian Objects With...Sérgio Sacani
We are conducting a survey for distant solar system objects beyond the Kuiper
Belt edge ( 50 AU) with new wide-field cameras on the Subaru and CTIO tele-
scopes. We are interested in the orbits of objects that are decoupled from the
giant planet region in order to understand the structure of the outer solar sys-
tem, including whether a massive planet exists beyond a few hundred AU as first
reported in Trujillo and Sheppard (2014). In addition to discovering extreme
trans-Neptunian objects detailed elsewhere, we have found several objects with
high perihelia (q > 40 AU) that differ from the extreme and inner Oort cloud
objects due to their moderate semi-major axes (50 < a < 100 AU) and eccen-
tricities (e . 0.3). Newly discovered objects 2014 FZ71 and 2015 FJ345 have
the third and fourth highest perihelia known after Sedna and 2012 VP113, yet
their orbits are not nearly as eccentric or distant. We found several of these high
perihelion but moderate orbit objects and observe that they are mostly near Nep-
tune mean motion resonances and have significant inclinations (i > 20 degrees).
These moderate objects likely obtained their unusual orbits through combined
interactions with Neptune’s mean motion resonances and the Kozai resonance,
similar to the origin scenarios for 2004 XR190. We also find the distant 2008
ST291 has likely been modified by the MMR+KR mechanism through the 6:1
Neptune resonance. We discuss these moderately eccentric, distant objects along
with some other interesting low inclination outer classical belt objects like 2012
FH84 discovered in our ongoing survey.
A giant ring_like_structure_at_078_z_086_displayed_by_gr_bsSérgio Sacani
This document describes the discovery of a giant ring-like structure in the observable universe displayed by 9 gamma ray bursts (GRBs) between redshifts of 0.78 and 0.86. The ring has a diameter of 1720 Mpc, over five times larger than the expected transition scale to homogeneity. The ring lies at a distance of 2770 Mpc with major and minor diameters of 43° and 30°, respectively. The probability of this structure occurring by random chance is calculated to be 2 × 10-6. This ring-shaped feature contradicts the cosmological principle of large-scale homogeneity and isotropy, and the physical mechanism responsible is unknown.
The kepler 10_planetary_system_revisited_by_harpsSérgio Sacani
This document discusses observations of the Kepler-10 planetary system made with the HARPS-N spectrograph. The observations resulted in improved precision for the masses of Kepler-10b and Kepler-10c. Kepler-10b's mass was determined to be 3.33 ± 0.49 Earth masses, confirming its rocky composition. Kepler-10c's mass of 17.2 ± 1.9 Earth masses makes it the first strong evidence of a class of more massive solid Neptune-sized planets with longer orbital periods. The improved mass measurements will help constrain models of the internal structure and composition of these planets.
Predictions of the_atmospheric_composition_of_gj_1132_bSérgio Sacani
GJ 1132 b is a nearby Earth-sized exoplanet transiting an M dwarf, and is amongst the most highly
characterizable small exoplanets currently known. In this paper we study the interaction of a magma
ocean with a water-rich atmosphere on GJ 1132b and determine that it must have begun with more
than 5 wt% initial water in order to still retain a water-based atmosphere. We also determine the
amount of O2
that can build up in the atmosphere as a result of hydrogen dissociation and loss.
We find that the magma ocean absorbs at most ∼ 10% of the O2 produced, whereas more than
90% is lost to space through hydrodynamic drag. The most common outcome for GJ 1132 b from our
simulations is a tenuous atmosphere dominated by O2
, although for very large initial water abundances
atmospheres with several thousands of bars of O2
are possible. A substantial steam envelope would
indicate either the existence of an earlier H2
envelope or low XUV flux over the system’s lifetime. A
steam atmosphere would also imply the continued existence of a magma ocean on GJ 1132 b. Further
modeling is needed to study the evolution of CO2
or N2
-rich atmospheres on GJ 1132 b.
This pilot survey used modest aperture telescopes to image 8 nearby spiral galaxies in order to search for stellar tidal streams. Ultra-deep imaging revealed 6 previously undetected extensive (up to 30 kpc) stellar structures likely from tidally disrupted satellites. A diversity of tidal feature morphologies was found, including great circle-like streams, remote shells, and jets emerging from disks. Simulations predict tidal debris should be common and match the observed variety, providing evidence minor mergers have shaped disk galaxies since z=1.
A super earth transiting a naked-eye starSérgio Sacani
This document summarizes the detection of transits of an exoplanet orbiting the star 55 Cnc, using photometry from the MOST space telescope. The transits match the period, phase, duration, and depth predicted for the innermost planet of 55 Cnc, designated 55 Cnc e. Analysis of the transit data indicates the planet has a mass of 8.57 Earth masses, a radius of 1.63 Earth radii, and a dense composition of rock and iron. This makes 55 Cnc e similar to other dense super-Earth exoplanets in short orbits, unlike lower-density super-Earths further from their stars. The brightness of 55 Cnc will enable further study of this transiting exoplanet system.
The green valley_is_a_red_herring_galaxy_zoo_reveals_two_evolutionary_pathwaysSérgio Sacani
This document summarizes research using data from Galaxy Zoo, SDSS, and GALEX to study how star formation is quenched in low-redshift galaxies. The key findings are:
1) Taking galaxy morphology into account, the "green valley" is not a single transitional state, as was previously thought.
2) Only a small population of blue early-type galaxies rapidly transition across the green valley as their morphology transforms from disk to spheroid and star formation is quenched quickly.
3) The majority of blue star-forming galaxies have significant disks and retain their late-type morphology as their star formation rates decline very slowly.
4) Different evolutionary pathways are observed for early- and late-type
Kinematics and simulations_of_the_stellar_stream_in_the_halo_of_the_umbrella_...Sérgio Sacani
This document summarizes a study of the stellar stream and substructures around the Umbrella Galaxy (NGC 4651). Deep imaging and spectroscopy were used to characterize the properties and kinematics of the stream. Tracer objects like globular clusters and planetary nebulae were identified and found to delineate a kinematically cold feature in position-velocity space. Dynamical modeling suggests the stream originated from the tidal disruption of a dwarf galaxy on a highly eccentric orbit about 6-10 billion years ago. This work demonstrates the feasibility of using discrete tracers to recover the kinematics and model the dynamics of low surface brightness stellar streams around distant galaxies.
The behaviour of_dark_matter_associated_with_4_bright_cluster_galaxies_in_the...Sérgio Sacani
The document presents new Hubble Space Telescope imaging and Very Large Telescope integral-field spectroscopy of galaxy cluster Abell 3827. It finds that each of the four central galaxies in the cluster's 10 kpc core retains an associated massive dark matter halo. At least one galaxy is offset from its dark matter halo, with an offset of 1.62+0.50−0.47 kpc, within the uncertainties of mass modeling and statistical errors. If interpreted as evidence for self-interacting dark matter, this offset implies a dark matter self-interaction cross-section of (1.7 ± 0.7)×10−4 cm2/g.
This document summarizes the discovery of a stellar tidal stream around the nearby dwarf galaxy NGC 4449 using deep imaging. The stream implies a massive dwarf spheroidal progenitor that is disrupting and depositing stars into NGC 4449's halo. The luminosity ratio between the two galaxies is around 1:50, but their dynamical mass ratio including dark matter may be 1:10-1:5. This "stealth merger" suggests that satellite accretion can significantly build up stellar halos of low-mass galaxies and possibly trigger their starbursts.
The fornax deep_survey_with_vst_i_the_extended_and_diffuse_stellar_halo_of_ng...Sérgio Sacani
We have started a new deep, multi-imaging survey of the Fornax cluster, dubbed Fornax Deep
Survey (FDS), at the VLT Survey Telescope. In this paper we present the deep photometry inside
two square degrees around the bright galaxy NGC 1399 in the core of the cluster. We found that
the core of the Fornax cluster is characterised by a very extended and diffuse envelope surrounding
the luminous galaxy NGC 1399: we map the surface brightness out to 33 arcmin (∼ 192 kpc)
from the galaxy center and down to μg ∼ 31 mag arcsec−2 in the g band. The deep photometry
allows us to detect a faint stellar bridge in the intracluster region on the west side of NGC 1399
and towards NGC 1387. By analyzing the integrated colors of this feature, we argue that it
could be due to the ongoing interaction between the two galaxies, where the outer envelope of
NGC 1387 on its east side is stripped away. By fitting the light profile, we found that exists a
physical break radius in the total light distribution at R = 10 arcmin (∼ 58 kpc) that sets the
transition region between the bright central galaxy and the outer exponential halo, and that the
stellar halo contributes for 60% of the total light of the galaxy (Sec. 3.5). We discuss the main
implications of this work on the build-up of the stellar halo at the center of the Fornax cluster.
By comparing with the numerical simulations of the stellar halo formation for the most massive
BCGs (i.e. 13 < logM200/M⊙ < 14), we find that the observed stellar halo mass fraction is
consistent with a halo formed through the multiple accretion of progenitors with stellar mass in
the range 108 − 1011 M⊙. This might suggest that the halo of NGC 1399 has also gone through
a major merging event. The absence of a significant number of luminous stellar streams and
tidal tails out to 192 kpc suggests that the epoch of this strong interaction goes back to an early
formation epoch. Therefore, differently from the Virgo cluster, the extended stellar halo around
NGC 1399 is characterised by a more diffuse and well-mixed component, including the ICL.
The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in th...GOASA
HR 5171 A exhibits a complex appearance based on AMBER/VLTI observations. The observations reveal an unusually large star of approximately 1315 solar radii at a distance of 3.6 kiloparsecs. The source is surrounded by an extended nebula. The observations also show a high level of asymmetry in the brightness distribution, which is attributed to the discovery of a companion star located in front of the primary. Analysis of visual photometry data indicates the system has an orbital period of 1304 days, providing evidence it is a contact or over-contact eclipsing binary. Modeling suggests a total system mass of 39-79 solar masses and a high mass ratio of at least 10 for the companion. The discovery of the
This document presents an analysis of transit spectroscopy observations of three exoplanets - WASP-12 b, WASP-17 b, and WASP-19 b - using the Wide Field Camera 3 instrument on the Hubble Space Telescope. The observations achieved almost photon-limited precision but uncertainties in the transit depths were increased by the uneven sampling of the light curves. The final transit spectra for all three planets are consistent with the presence of a water absorption feature at 1.4 microns, though the amplitude is smaller than expected from previous Spitzer observations possibly due to hazes. Due to degeneracies between models, the data cannot unambiguously constrain the atmospheric compositions without additional observations.
SPECTROSCOPIC CONFIRMATION OF THE EXISTENCE OF LARGE, DIFFUSE GALAXIES IN THE...Sérgio Sacani
We recently identified a population of low surface brightness objects in the field of the z = 0.023 Coma cluster,
using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these “ultradiffuse
galaxies” (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption
features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz=6280±120 km s−1 is within the 1σ velocity dispersion of the Coma cluster. The galaxy has an effective
radius of 4.3 ± 0.3 kpc and a Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ∼2re. We show that UDGs are located in a previously
sparsely populated region of the size—magnitude plane of quiescent stellar systems, as they are ∼6 mag fainter
than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent
galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are
needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same
luminosity or to those of galaxies with the same size.
The article discusses two gaseous regions discovered that have a chemical composition close to that of the early universe before the first stars formed. This finding demonstrates that metals dispersed unevenly throughout the universe, with implications for when the first generation of stars could have formed. The early universe began with hydrogen, helium, and trace amounts of lithium produced by Big Bang nucleosynthesis. Later, the first stars enriched and ionized the universe with heavier elements, though it appears this process was nonuniform based on the discovery of pockets of nearly pristine gas.
Ages and stratigraphy of lunar mare basalts a synthesisSérgio Sacani
This document summarizes ages and stratigraphy of lunar mare basalts based on a synthesis of crater counting efforts over more than 10 years. The key points are:
1) Lunar volcanism was active for almost 3 billion years, starting around 3.9-4 billion years ago and ceasing around 1.2 billion years ago.
2) Most basalts erupted during the late Imbrian period around 3.6-3.8 billion years ago.
3) Significantly fewer basalts were emplaced during the Eratosthenian period.
4) Basalts of possible Copernican age have only been found in limited areas of Oceanus Procellarum.
This document summarizes a study that estimates the dynamical surface mass density between 1.5 and 4 kpc from the Galactic plane using kinematics of thick disk stars. The authors derive an exact analytical expression for the surface density based on assumptions about the stellar population and Galactic potential. Their expression matches expectations of visible mass alone, with no evidence for additional dark matter required. They extrapolate a local dark matter density of 0±1 mM⊙ pc−3, excluding all current spherical dark matter halo models at over 4σ confidence. Only a highly prolate dark matter halo could potentially reconcile the observations with models, but this is unlikely according to ΛCDM.
Lunar skylight polarization signal polluted by urban lightingSérgio Sacani
1. This study found that urban skyglow has a greater degree of linear polarization (8.6%) than expected, which diminishes the natural polarization signal of scattered moonlight.
2. Measurements taken in Berlin found the lunar polarization signal was clearly visible in a rural area 28 km from the city center (29.2%), but was reduced within the city itself (11.3%).
3. Without the influence of skyglow, the lunar polarization signal would likely exceed 50%. This indicates that nocturnal animal navigation systems relying on perceiving polarized moonlight likely fail in highly light-polluted urban areas.
The small blue_straggler_star_population_in_the_dense_galactic_globular_clust...Sérgio Sacani
This document summarizes a study of blue straggler star (BSS) populations in the globular cluster NGC 6752. The study used high-resolution Hubble Space Telescope images of the cluster core and wide-field ground-based images covering the outer regions. A total of 28 BSS candidates were identified in the HST images, and an additional 15 BSS candidates were found in the outer regions from the ground-based images. The radial distribution of BSS will be analyzed to understand the dynamical evolution of the cluster.
This document summarizes the results of a 180 ks Chandra-LETGS observation of Mrk 509 as part of a larger multi-wavelength campaign. The observation detected several absorption features in the X-ray spectrum originating from an ionized absorber, including ions with three distinct ionization degrees. The lowest ionized component is slightly redshifted and not in pressure equilibrium with the others, likely belonging to the host galaxy's interstellar medium. The other two components are outflowing at velocities of around -200 and -455 km/s. Simultaneous HST-COS observations detected 13 UV kinematic components, and at least three can be associated with the X-ray components, providing evidence that the UV and X-
This document describes the Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES) catalog, which contains 1342 very wide (projected separation >500 AU), low-mass (at least one mid-K to mid-M dwarf component) common proper motion pairs identified from the Sloan Digital Sky Survey. The catalog was constructed using astrometry, photometry, and proper motions to identify pairs with a probability of chance alignment ≤0.05. The catalog is intended to be a resource for detailed study of low-mass binary star systems and preliminary results describe the properties and characteristics of the wide, low-mass pairs.
The galaxy morphology–density relation in the EAGLE simulationSérgio Sacani
This study uses a convolutional neural network trained on observed galaxy images to classify galaxies in the EAGLE cosmological hydrodynamical simulation by visual morphology. It finds that EAGLE reproduces both the observed relationships between galaxy morphology and local density (the morphology-density relation), and between morphology and galaxy mass. Through analysis of the simulations, the key processes identified as driving the morphology-density relation are: (1) the transformation of disc galaxies into lenticular galaxies via gas stripping in dense environments, (2) the formation of lenticular galaxies through merger-induced black hole feedback in less dense environments, and (3) the increasing fraction of more elliptical, high-mass galaxies at higher densities.
This document summarizes a study that uses cosmological simulations to model the formation of stellar halos around galaxies via the tidal disruption of accreted dwarf galaxies. The simulations follow the dynamical evolution and disruption of satellites from high redshift in a fully cosmological setting. The simulations produce stellar halos with masses and density profiles consistent with observations of the Milky Way and M31. The stellar halos show complex structures composed of well-mixed components, tidal streams, shells, and other substructures rather than smooth distributions.
Star formation at the smallest scales; A JWST study of the clump populations ...Sérgio Sacani
We present the clump populations detected in 18 lensed galaxies at redshifts 1 to 8.5 within the lensing cluster field SMACS0723.
The recent JWST Early Release Observations of this poorly known region of the sky have revealed numerous point-like sources
within and surrounding their host galaxies, undetected in the shallower HST images. We use JWST multiband photometry and
the lensing model of this galaxy cluster to estimate the intrinsic sizes and magnitudes of the stellar clumps. We derive optical
restframe effective radii from <10 to hundreds pc and masses ranging from ∼ 105
to 109 M, overlapping with massive star
clusters in the local universe. Clump ages range from 1 Myr to 1 Gyr. We compare the crossing time to the age of the clumps
and determine that between 45 and 60 % of the detected clumps are consistent with being gravitationally bound. On average,
the dearth of Gyr old clumps suggests that the dissolution time scales are shorter than 1 Gyr. We see a significant increase in the
luminosity (mass) surface density of the clumps with redshift. Clumps in reionisation era galaxies have stellar densities higher
than star clusters in the local universe. We zoom in into single galaxies at redshift < 6 and find for two galaxies, the Sparkler and
the Firework, that their star clusters/clumps show distinctive colour distributions and location surrounding their host galaxy that
are compatible with being accredited or formed during merger events. The ages of some of the compact clusters are between
1 and 4 Gyr, e.g., globular cluster precursors formed around 9-12 Gyr ago. Our study, conducted on a small sample of galaxies,
shows the potential of JWST observations for understanding the conditions under which star clusters form in rapidly evolving
galaxies.
This document summarizes evidence that submillimeter galaxies (SMGs) at redshift 3-6 may be progenitors of compact quiescent galaxies observed at redshift 2. It compares properties of a sample of z~2 quiescent galaxies with a statistical sample of z>3 SMGs in the COSMOS field. It finds that the formation redshifts of the z~2 galaxies match the observed redshift distribution of z>3 SMGs. It also finds that the space densities and properties such as sizes, stellar masses, and internal velocities of the two populations are consistent with an evolutionary connection, assuming SMG starbursts have a duty cycle of 42+40 Myr. This suggests SMGs may represent an early burst
This study analyzed high-resolution spectra of 125 compact stellar systems (CSSs) in the giant elliptical galaxy NGC 5128 to measure their radial velocities and velocity dispersions. Combining these measurements with structural parameters from imaging, dynamical masses were derived for 112 CSSs, including 89 for the first time. Two distinct sequences were found in the dynamical mass-to-light ratio vs dynamical mass plane, which can be approximated by power laws. The shallower sequence corresponds to bright globular clusters, while the steeper relation appears to be populated by objects requiring significant dark matter such as central black holes or concentrated dark matter. This suggests different formation histories for these CSSs compared to classical globular clusters in NGC 5128 and
Relativistic collapse and explosion of rotating supermassive stars with therm...Sérgio Sacani
1) The document describes general relativistic simulations of collapsing supermassive stars with and without rotation using a numerical code called Nada.
2) The simulations include effects of gas pressure, radiation, electron-positron pairs, and thermonuclear energy from hydrogen and helium burning.
3) Objects with a mass of around 5×105 solar masses explode if non-rotating with a metallicity over 0.007, while rotation lowers the threshold to 0.001. More massive objects have a higher critical metallicity for explosion.
The SAMI Galaxy Sur v ey: galaxy spin is more strongly correlated with stella...Sérgio Sacani
We use the SAMI Galaxy Surv e y to examine the drivers of galaxy spin, λR e , in a multidimensional parameter space including stellar mass, stellar population age (or specific star formation rate), and various environmental metrics (local density, halo mass, satellite versus central). Using a partial correlation analysis, we consistently find that age or specific star formation rate is the primary parameter correlating with spin. Light-weighted age and specific star formation rate are more strongly correlated with spin than mass-weighted age. In fact, across our sample, once the relation between light-weighted age and spin is accounted for, there is no significant residual correlation between spin and mass, or spin and environment. This result is strongly suggestive that the present-day environment only indirectly influences spin, via the removal of gas and star formation quenching. That is, environment affects age, then age affects spin. Older galaxies then have lower spin, either due to stars being born dynamically hotter at high redshift, or due to secular heating. Our results appear to rule out environmentally dependent dynamical heating (e.g. g alaxy–g alaxy interactions) being important, at least within 1 R e where our kinematic measurements are made. The picture is more complex when we only consider high-mass galaxies ( M ∗ ≳ 10 11 M ). While the age-spin relation is still strong for these high-mass galaxies, there is a residual environmental trend with central galaxies preferentially having lower spin, compared to satellites of the same age and mass. We argue that this trend is likely due to central galaxies being a preferred location for mergers.
Disks of Stars in the Galactic Center Triggered by Tidal Disruption EventsSérgio Sacani
This document proposes that tidal disruption events (TDEs) from wandering stars could trigger episodes of positive star formation feedback in the Galactic Center, providing an explanation for the observed disks of young stars near Sgr A*. When a star is tidally disrupted by the supermassive black hole, the resulting jet compresses gas clouds to densities high enough to resist tidal forces and form stars within the disk plane perpendicular to the jet. The estimated rate of jetted TDEs is consistent with the age of the disk stars. This mechanism predicts a random orientation for each disk and the potential for multiple misaligned disks from separate TDE events.
The document analyzes the merger fractions of radio-loud and radio-quiet active galactic nuclei (AGN) at z > 1 using new Hubble Space Telescope (HST) samples. It finds that 92% (+8%/-14%) of radio-loud galaxies at z > 1 show evidence of recent or ongoing mergers, while only 38% (+16%/-15%) of matched radio-quiet samples exhibit mergers. This provides strong evidence that mergers are the triggering mechanism for radio-loud AGN phenomena like relativistic jet launching, and that major black hole mergers may spin up central supermassive black holes in these objects.
DYNAMICAL ANALYSIS OF THE DARK MATTER AND CENTRAL BLACK HOLE MASS IN THE DWAR...Sérgio Sacani
We measure the central kinematics for the dwarf spheroidal galaxy Leo I using integrated-light measurements and
previously published data. We find a steady rise in the velocity dispersion from 30000 into the center. The integratedlight kinematics provide a velocity dispersion of 11.76±0.66 km s−1
inside 7500. After applying appropriate corrections
to crowding in the central regions, we achieve consistent velocity dispersion values using velocities from individual stars.
Crowding corrections need to be applied when targeting individual stars in high density stellar environments. From
integrated light, we measure the surface brightness profile and find a shallow cusp towards the center. Axisymmetric,
orbit-based models measure the stellar mass-to-light ratio, black hole mass and parameters for a dark matter halo. At
large radii it is important to consider possible tidal effects from the Milky Way so we include a variety of assumptions
regarding the tidal radius. For every set of assumptions, models require a central black hole consistent with a mass
3.3 ± 2×106 M. The no-black-hole case for any of our assumptions is excluded at over 95% significance, with
6.4 < ∆χ
2 < 14. A black hole of this mass would have significant effect on dwarf galaxy formation and evolution.
The dark halo parameters are heavily affected by the assumptions for the tidal radii, with the circular velocity only
constrained to be above 30 km s−1
. Reasonable assumptions for the tidal radius result in stellar orbits consistent with
an isotropic distribution in the velocities. These more realistic models only show strong constraints for the mass of
the central black hole.
A spectroscopic sample_of_massive_galaxiesSérgio Sacani
This document describes a study of 16 massive galaxies at z ~ 2 selected from the 3D-HST spectroscopic survey based on the detection of a strong 4000 Angstrom break in their spectra. Spectroscopy and imaging from HST/WFC3 are used to determine accurate redshifts, stellar population properties, and structural parameters. The sample significantly increases the number of spectroscopically confirmed evolved galaxies at z ~ 2 with robust size measurements. The analysis populates the mass-size relation and finds it is consistent with local relations but with smaller sizes by a factor of 2-3. A model is presented where the observed size evolution is explained by quenching of increasingly larger star-forming galaxies at a rate set by
Galaxy interactions are the dominant trigger for local type 2 quasarsSérgio Sacani
The triggering mechanism for the most luminous, quasar-like active galactic nuclei (AGN) remains a source of debate, with
some studies favouring triggering via galaxy mergers, but others finding little evidence to support this mechanism. Here, we
present deep Isaac Newton Telescope/Wide Field Camera imaging observations of a complete sample of 48 optically selected
type 2 quasars – the QSOFEED sample (L[O III] > 108.5 L; z < 0.14). Based on visual inspection by eight classifiers, we find
clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with
65+6
−7 per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared
with only 22+5
−4 per cent of a stellar-mass- and redshift-matched comparison sample of non-AGN galaxies – a 5σ difference. The
type 2 quasar hosts are a factor of 3.0+0.5 −0.8 more likely to be morphologically disturbed than their matched non-AGN counterparts,
similar to our previous results for powerful 3CR radio AGN of comparable [O III] emission-line luminosity and redshift. In
contrast to the idea that quasars are triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only become
visible post-coalescence, the majority of morphologically disturbed type 2 quasar sources in our sample are observed in the
pre-coalescence phase (61+8
−9 per cent). We argue that much of the apparent ambiguity that surrounds observational results in this
field is a result of differences in the surface brightness depths of the observations, combined with the effects of cosmological
surface brightness dimming.
The network of filaments with embedded clusters surrounding voids, which has been seen in maps derived from
redshift surveys and reproduced in simulations, has been referred to as the cosmic web. A complementary
description is provided by considering the shear in the velocity field of galaxies. The eigenvalues of the shear
provide information regarding whether or not a region is collapsing in three dimensions, which is the condition for
a knot, expanding in three dimensions, which is the condition for a void, or in the intermediate condition of a
filament or sheet. The structures that are quantitatively defined by the eigenvalues can be approximated by isocontours
that provide a visual representation of the cosmic velocity (V) web. The current application is based on
radial peculiar velocities from the Cosmicflows-2 collection of distances. The three-dimensional velocity field is
constructed using the Wiener filter methodology in the linear approximation. Eigenvalues of the velocity shear are
calculated at each point on a grid. Here, knots and filaments are visualized across a local domain of
diameter ~0.1c
Candels the correlation_between_galaxy_morphology_and_star_formation_activity...Sérgio Sacani
This document summarizes a study investigating the relationship between galaxy morphology and star formation activity at z ~ 2 using a sample of 1,671 galaxies from CANDELS images in the GOODS-South field. The sample separates into massive, red, passive galaxies and less massive, blue, star-forming galaxies, correlating well with morphological properties. Star-forming galaxies show a variety of morphologies including clumpy structures and bulges mixed with faint disks, while passive galaxies often have compact morphologies resembling local spheroids. Similar trends are seen in local massive galaxies, suggesting the Hubble sequence was in place by z ~ 2.
Lenz et al. discovered that soft particles at the nano- and microscale can form "clumpy crystals" where particles partially overlap and form regular lattices of clumps. This challenges ideas that soft materials will behave similarly to atomic and molecular systems. The discovery provides another example of how soft materials display unconventional behavior. An analysis of the satellites orbiting Andromeda found that about half are rotating coherently in a thin planar structure, providing a new constraint on galaxy formation theories. Further evidence suggests organized planar distributions of satellites may be common for nearby galaxy groups. The findings compound issues with the number of predicted versus observed satellites and suggest the structures themselves are not ancient.
WHAT CAN WE DEDUCE FROM STUDIES OF NEARBY GALAXY POPULATIONS?CosmoAIMS Bassett
Studies of nearby galaxy populations using large optical surveys like SDSS have provided insights into galaxy formation and evolution. Key findings include identifying characteristic scales where baryon conversion peaks at halo masses of ~10^12 solar masses and galaxies transition from blue to red at stellar masses of ~10^10 solar masses. While surveys have constrained stellar populations and traced dark matter halos, they have not well constrained gas accretion onto galaxies, gas outflows, or the influence of black holes on galaxy evolution.
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.
The gaia eso_survey_stellar_content_and_elemental_abundances_in_the_massive_c...Sérgio Sacani
Estudo sobre o conteúdo estelar e os elementos que estão presentes no aglomerado estelar aberto NGC 6705, também conhecido como Aglomerado do Pato Selvagem.
Fizzy Super-Earths: Impacts of Magma Composition on the Bulk Density and Stru...Sérgio Sacani
Lava worlds are a potential emerging population of Super-Earths that are on close-in orbits around their host stars,
with likely partially molten mantles. To date, few studies have addressed the impact of magma on the observed
properties of a planet. At ambient conditions, magma is less dense than solid rock; however, it is also more
compressible with increasing pressure. Therefore, it is unclear how large-scale magma oceans affect planet
observables, such as bulk density. We update ExoPlex, a thermodynamically self-consistent planet interior
software, to include anhydrous, hydrous (2.2 wt% H2O), and carbonated magmas (5.2 wt% CO2). We find that
Earth-like planets with magma oceans larger than ∼1.5 R⊕ and ∼3.2 M⊕ are modestly denser than an equivalentmass
solid planet. From our model, three classes of mantle structures emerge for magma ocean planets: (1) a
mantle magma ocean, (2) a surface magma ocean, and (3) one consisting of a surface magma ocean, a solid rock
layer, and a basal magma ocean. The class of planets in which a basal magma ocean is present may sequester
dissolved volatiles on billion-year timescales, in which a 4 M⊕ mass planet can trap more than 130 times the mass
of water than in Earth’s present-day oceans and 1000 times the carbon in the Earth’s surface and crust.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
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.
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.
Fueling AI with Great Data with Airbyte WebinarZilliz
This talk will focus on how to collect data from a variety of sources, leveraging this data for RAG and other GenAI use cases, and finally charting your course to productionalization.
For the full video of this presentation, please visit: https://www.edge-ai-vision.com/2024/06/building-and-scaling-ai-applications-with-the-nx-ai-manager-a-presentation-from-network-optix/
Robin van Emden, Senior Director of Data Science at Network Optix, presents the “Building and Scaling AI Applications with the Nx AI Manager,” tutorial at the May 2024 Embedded Vision Summit.
In this presentation, van Emden covers the basics of scaling edge AI solutions using the Nx tool kit. He emphasizes the process of developing AI models and deploying them globally. He also showcases the conversion of AI models and the creation of effective edge AI pipelines, with a focus on pre-processing, model conversion, selecting the appropriate inference engine for the target hardware and post-processing.
van Emden shows how Nx can simplify the developer’s life and facilitate a rapid transition from concept to production-ready applications.He provides valuable insights into developing scalable and efficient edge AI solutions, with a strong focus on practical implementation.
Cosa hanno in comune un mattoncino Lego e la backdoor XZ?Speck&Tech
ABSTRACT: A prima vista, un mattoncino Lego e la backdoor XZ potrebbero avere in comune il fatto di essere entrambi blocchi di costruzione, o dipendenze di progetti creativi e software. La realtà è che un mattoncino Lego e il caso della backdoor XZ hanno molto di più di tutto ciò in comune.
Partecipate alla presentazione per immergervi in una storia di interoperabilità, standard e formati aperti, per poi discutere del ruolo importante che i contributori hanno in una comunità open source sostenibile.
BIO: Sostenitrice del software libero e dei formati standard e aperti. È stata un membro attivo dei progetti Fedora e openSUSE e ha co-fondato l'Associazione LibreItalia dove è stata coinvolta in diversi eventi, migrazioni e formazione relativi a LibreOffice. In precedenza ha lavorato a migrazioni e corsi di formazione su LibreOffice per diverse amministrazioni pubbliche e privati. Da gennaio 2020 lavora in SUSE come Software Release Engineer per Uyuni e SUSE Manager e quando non segue la sua passione per i computer e per Geeko coltiva la sua curiosità per l'astronomia (da cui deriva il suo nickname deneb_alpha).
Ivanti’s Patch Tuesday breakdown goes beyond patching your applications and brings you the intelligence and guidance needed to prioritize where to focus your attention first. Catch early analysis on our Ivanti blog, then join industry expert Chris Goettl for the Patch Tuesday Webinar Event. There we’ll do a deep dive into each of the bulletins and give guidance on the risks associated with the newly-identified vulnerabilities.
In the rapidly evolving landscape of technologies, XML continues to play a vital role in structuring, storing, and transporting data across diverse systems. The recent advancements in artificial intelligence (AI) present new methodologies for enhancing XML development workflows, introducing efficiency, automation, and intelligent capabilities. This presentation will outline the scope and perspective of utilizing AI in XML development. The potential benefits and the possible pitfalls will be highlighted, providing a balanced view of the subject.
We will explore the capabilities of AI in understanding XML markup languages and autonomously creating structured XML content. Additionally, we will examine the capacity of AI to enrich plain text with appropriate XML markup. Practical examples and methodological guidelines will be provided to elucidate how AI can be effectively prompted to interpret and generate accurate XML markup.
Further emphasis will be placed on the role of AI in developing XSLT, or schemas such as XSD and Schematron. We will address the techniques and strategies adopted to create prompts for generating code, explaining code, or refactoring the code, and the results achieved.
The discussion will extend to how AI can be used to transform XML content. In particular, the focus will be on the use of AI XPath extension functions in XSLT, Schematron, Schematron Quick Fixes, or for XML content refactoring.
The presentation aims to deliver a comprehensive overview of AI usage in XML development, providing attendees with the necessary knowledge to make informed decisions. Whether you’re at the early stages of adopting AI or considering integrating it in advanced XML development, this presentation will cover all levels of expertise.
By highlighting the potential advantages and challenges of integrating AI with XML development tools and languages, the presentation seeks to inspire thoughtful conversation around the future of XML development. We’ll not only delve into the technical aspects of AI-powered XML development but also discuss practical implications and possible future directions.
GraphRAG for Life Science to increase LLM accuracyTomaz Bratanic
GraphRAG for life science domain, where you retriever information from biomedical knowledge graphs using LLMs to increase the accuracy and performance of generated answers
Generating privacy-protected synthetic data using Secludy and MilvusZilliz
During this demo, the founders of Secludy will demonstrate how their system utilizes Milvus to store and manipulate embeddings for generating privacy-protected synthetic data. Their approach not only maintains the confidentiality of the original data but also enhances the utility and scalability of LLMs under privacy constraints. Attendees, including machine learning engineers, data scientists, and data managers, will witness first-hand how Secludy's integration with Milvus empowers organizations to harness the power of LLMs securely and efficiently.
UiPath Test Automation using UiPath Test Suite series, part 6DianaGray10
Welcome to UiPath Test Automation using UiPath Test Suite series part 6. In this session, we will cover Test Automation with generative AI and Open AI.
UiPath Test Automation with generative AI and Open AI webinar offers an in-depth exploration of leveraging cutting-edge technologies for test automation within the UiPath platform. Attendees will delve into the integration of generative AI, a test automation solution, with Open AI advanced natural language processing capabilities.
Throughout the session, participants will discover how this synergy empowers testers to automate repetitive tasks, enhance testing accuracy, and expedite the software testing life cycle. Topics covered include the seamless integration process, practical use cases, and the benefits of harnessing AI-driven automation for UiPath testing initiatives. By attending this webinar, testers, and automation professionals can gain valuable insights into harnessing the power of AI to optimize their test automation workflows within the UiPath ecosystem, ultimately driving efficiency and quality in software development processes.
What will you get from this session?
1. Insights into integrating generative AI.
2. Understanding how this integration enhances test automation within the UiPath platform
3. Practical demonstrations
4. Exploration of real-world use cases illustrating the benefits of AI-driven test automation for UiPath
Topics covered:
What is generative AI
Test Automation with generative AI and Open AI.
UiPath integration with generative AI
Speaker:
Deepak Rai, Automation Practice Lead, Boundaryless Group and UiPath MVP
Digital Marketing Trends in 2024 | Guide for Staying AheadWask
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HCL Notes and Domino License Cost Reduction in the World of DLAUpanagenda
Webinar Recording: https://www.panagenda.com/webinars/hcl-notes-and-domino-license-cost-reduction-in-the-world-of-dlau/
The introduction of DLAU and the CCB & CCX licensing model caused quite a stir in the HCL community. As a Notes and Domino customer, you may have faced challenges with unexpected user counts and license costs. You probably have questions on how this new licensing approach works and how to benefit from it. Most importantly, you likely have budget constraints and want to save money where possible. Don’t worry, we can help with all of this!
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Monitoring and Managing Anomaly Detection on OpenShift.pdfTosin Akinosho
Monitoring and Managing Anomaly Detection on OpenShift
Overview
Dive into the world of anomaly detection on edge devices with our comprehensive hands-on tutorial. This SlideShare presentation will guide you through the entire process, from data collection and model training to edge deployment and real-time monitoring. Perfect for those looking to implement robust anomaly detection systems on resource-constrained IoT/edge devices.
Key Topics Covered
1. Introduction to Anomaly Detection
- Understand the fundamentals of anomaly detection and its importance in identifying unusual behavior or failures in systems.
2. Understanding Edge (IoT)
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3. What is ArgoCD?
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7. What is Prometheus?
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- Learn how to configure Camel K for seamless data pipeline integrations in your anomaly detection workflow.
11. What is a Jupyter Notebook?
- Overview of Jupyter Notebooks, an open-source web application for creating and sharing documents with live code, equations, visualizations, and narrative text.
12. Jupyter Notebooks with Code Examples
- Hands-on examples and code snippets in Jupyter Notebooks to help you implement and test anomaly detection models.
Monitoring and Managing Anomaly Detection on OpenShift.pdf
1108.2508v1
1. accepted to the Astrophysical Journal 10 August 2011
Preprint typeset using L TEX style emulateapj v. 8/13/10
A
THE MAJOR AND MINOR GALAXY MERGER RATES AT Z < 1.5
Jennifer M. Lotz1,2,3 , Patrik Jonsson4 , T.J. Cox5,6 , Darren Croton7 ,
Joel R. Primack8 , Rachel S. Somerville3, 9 , and Kyle Stewart10, 11
accepted to the Astrophysical Journal 10 August 2011
ABSTRACT
arXiv:1108.2508v1 [astro-ph.CO] 11 Aug 2011
Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates,
and a cosmologically-averaged ‘observability’ timescale Tobs (z) for identifying galaxy mergers. While
many have counted galaxy mergers using a variety of techniques, Tobs (z) for these techniques have
been poorly constrained. We address this problem by calibrating three merger rate estimators with a
suite of hydrodynamic merger simulations and three galaxy formation models. We estimate Tobs (z)
for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G − M20 ,
and (3) the morphology indicator asymmetry A. Then we apply these timescales to the observed
merger fractions at z < 1.5 from the recent literature. When our physically-motivated timescales
are adopted, the observed galaxy merger rates become largely consistent. The remaining differences
between the galaxy merger rates are explained by the differences in the range of mass-ratio measured
by different techniques and differing parent galaxy selection. The major merger rate per unit co-
moving volume for samples selected with constant number density evolves much more strongly with
redshift (∝ (1 + z)+3.0±1.1 ) than samples selected with constant stellar mass or passively evolving
luminosity (∝ (1 + z)+0.1±0.4 ). We calculate the minor merger rate (1:4 < Msat /Mprimary 1:10)
by subtracting the major merger rate from close pairs from the ‘total’ merger rate determined by
G − M20 . The implied minor merger rate is ∼ 3 times the major merger rate at z ∼ 0.7, and shows
little evolution with redshift.
Subject headings: galaxies:evolution – galaxies:high-redshift – galaxies:interacting – galaxies:structure
1. INTRODUCTION of dispersion-dominated spheroids (e.g. Toomre 1977;
The galaxy merger rate over cosmic time is one of White & Rees 1978; Kauffmann, White, & Guiderdoni
the fundamental measures of the evolution of galaxies. 1993; Mihos & Hernquist 1996; Sanders & Mirabel 1996;
Galaxies and the dark matter halos they live in must Somerville et al. 2001, 2008; di Matteo et al. 2008; Hop-
grow with time through mergers with other galaxies and kins et al. 2006, 2008). Because other physical processes
through the accretion of gas and dark matter from the are also at work, direct observations of the galaxy merg-
cosmic web. Over the past 10 billion years, the global ers are needed to understand their global importance to
star-formation rate density has declined by a factor of 10 galaxy evolution and assembly.
(e.g. Lilly et al. 1996; Madau et al. 1996; Hopkins & In a cold-dark matter dominated universe, massive
Beacom 2006) while the global stellar-mass density has structures are expected to grow hierarchically. Numeri-
increased by a factor of two (e.g. Rudnick et al. 2003; cal simulations consistently predict that the dark matter
Dickinson et al. 2003) . At the same time, massive galax- halo - halo merger rate per progenitor (or descendant)
ies have been transformed from rapidly star-forming disk halo at fixed halo mass changes rapidly with redshift
galaxies into quiescent bulge-dominated galaxies hosting ∼ (1 + z)2−3 (e.g. Gottl¨ber, Klypin, & Kratsvov 2001;
o
super-massive black holes (e.g. Bell et al. 2004; Faber Fakhouri & Ma 2008; Genel et al. 2009; Fakhouri, Ma,
et al. 2007; Brown et al. 2007). Galaxy mergers may & Boylan-Kolchin 2010). The dark matter merger rate
be an important process that drives galaxy assembly, scales with mass and mass ratio (Fakhouri & Ma 2008;
rapid star-formation at early times, the accretion of gas Fakhouri et al. 2010). More massive halos are rarer, but
onto central super-massive black holes, and the formation have more frequent merger rates per halo. Minor mergers
with mass ratios greater than 1:4 should be much more
1 National Optical Astronomical Observatories, 950 N. Cherry common per halo than major mergers with comparable
Avenue, Tucson, AZ 85719, USA
2 Leo Goldberg Fellow
mass halos.
3 Space Telescope Science Institute, 3700 San Martin Dr., Bal- However, the theoretical predictions for the galaxy
timore, MD 21218; lotz@stsci.edu merger rate remain highly uncertain (e.g. Jogee et al.
4 Harvard-Smithsonian Center for Astrophysics, Cambridge, 2009; see Hopkins et al. 2010a for a review). The pre-
MA, USA
5 Carnegie Observatories, Pasadena, CA, USA
dicted (1 + z)3 evolution in the dark-matter halo merger
6 Carnegie Fellow rate (per halo above a fixed total mass) does not auto-
7 Centre for Astrophysics & Supercomputing, Swinburne Uni- matically translate into a (1 + z)3 evolution in the galaxy
versity of Technology, Hawthorn, Australia merger rate (per galaxy above a fixed stellar mass) be-
8 Department of Physics, University of California, Santa Cruz,
USA
cause there is not a simple connection between observed
9 Department of Physics & Astronomy, Johns Hopkins Uni- galaxies and dark matter halos (e.g. Berrier et al. 2006;
versity, Baltimore, MD, USA Hopkins et al. 2010a, Moster et al. 2010). For exam-
10 Jet Propulsion Laboratory, Pasadena, CA, USA
11 NASA Postdoctoral Fellow
ple, the differences in the dark matter halo mass function
2. 2
and the galaxy stellar mass function at the high mass and range of merger properties, including progenitor mass,
low mass ends naturally produces a discrepancy between mass ratio, gas fraction, and orbital parameters (Cox et
the merger rates as a function of stellar and dark-matter al. 2006, 2008). Mass ratio and gas fraction are es-
mass and mass ratio. pecially important for interpreting the merger fractions
Many galaxy evolution models predict that the evolu- derived from morphological studies (Lotz et al. 2010a,
tion of major mergers of galaxies selected above a fixed b).
stellar mass (∼ 1010 M⊙ ) changes more modestly with The goal of this paper is to self-consistently determine
redshift ( ∼ (1 + z)1−2 ), but these can have an order of the observational galaxy merger rate at z < 1.5 from re-
magnitude variation in their normalizations (e.g. Jogee cent close pair and morphological studies. We weight the
et al. 2009; Hopkins et al. 2010a). The largest source of merger timescales from individual high-resolution simu-
theoretical uncertainty for semi-analytic galaxy evolution lations by the expected distribution of merger properties
models is the baryonic physics required to map galaxies (including gas fraction and mass ratio) to determine the
onto dark matter halos and sub-halos, and in turn, re- average observability timescale as a function of redshift
quired to match the observational selection of luminous for each method for finding galaxy mergers (close pairs,
merging galaxies (Hopkins et al. 2010a and references G − M20 , asymmetry). These new calculations of the av-
therein). In contrast, semi-empirical models which map erage observability timescales are crucial for interpreting
galaxies onto dark matter sub-halos by matching the ob- the observed galaxy merger fractions. We re-analyze the
served abundances of galaxies (per unit luminosity) to observations of galaxy mergers at z < 1.5 from the recent
the abundances of sub-halos (per unit mass) give reason- literature and derive new estimates of the galaxy merger
ably good predictions for clustering statistics by adopt- rate. We also carefully consider the effects of parent
ing fairly small dispersions in the mapping function (e.g. sample selection on the inferred evolution of the galaxy
Zheng et al. 2007, Conroy & Wechsler 2009) and give merger rate. We conclude that the differences in the
a only factor of two discrepancy in the predicted major observed galaxy merger fractions may be accounted for
merger rates (e.g. Hopkins et al. 2010b, Stewart et al. by the different observability timescales, different mass-
2009a). ratio sensitivities, and different parent galaxy selections.
Despite more than a decade of work, measurements of When these differences in the methodology are properly
the observed galaxy merger rate and its evolution with accounted for by adopting our derived timescales, we are
redshift has not converged. Current observations of the able to derive self-consistent estimates of the major and
fraction of bright/massive galaxies undergoing a merger minor galaxy merger rate and its evolution.
differ by an order of magnitude (Fig. 1), and estimates In §2, we define the volume-averaged galaxy merger
of the evolution in this merger fraction of at 0 < z < 1.5 rate, Γmerg , as the co-moving number density of on-going
vary from weak or no evolution (e.g. Bundy et al. 2004; galaxy merger events per unit time, and the fractional
Lin et al 2004; Lotz et al. 2008a; Jogee et al. 2009; galaxy merger rate, ℜmerg , as the number of galaxy
Bundy et al. 2009; Shi et al. 2009; de Ravel et al. 2009; merger events per unit time per selected galaxy. We dis-
Robaina et al 2010) to strong evolution ( ∼ (1 + z)3 ; Le cuss how to calculate Γmerg and ℜmerg using close pairs
Fe´re et al. 2000; Conselice et al. 2009; Cassata et al.
v and disturbed morphologies. In §3, we review the recent
2004; Kartaltepe et al. 2007; Rawat et al. 2008; Bridge literature estimates of the galaxy merger fraction using
et al. 2010; L´pez-Sanjuan et al. 2009 ). It has been
o quantitative morphology (G−M20 , asymmetry) and close
difficult to understand the source of these discrepancies, pairs of galaxies and discuss the importance of the parent
as the various studies often employ different criteria for sample selection. In §4, we review the results from the in-
counting galaxy mergers and different selections for the dividual high-resolution galaxy merger simulations. We
parent galaxy samples. present new calculations of the cosmologically-averaged
These observational studies identify galaxy merger merger observability timescale Tobs (z) for close pairs,
candidates either as galaxies in close pairs (in projected G−M20 , and asymmetry, using the distribution of galaxy
or real 3-D space) or galaxies with distorted morpholo- merger properties predicted by three different galaxy evo-
gies (measured quantitatively or by visual inspection). A lution models (Croton et al. 2006; Somerville et al. 2008;
key obstacle to the consistent measurement of the galaxy Stewart et al. 2009b). In §5, we derive the fractional
merger rate has been the poorly constrained timescales and volume-averaged major and minor galaxy merger
for detecting galaxy mergers selected by different meth- rates for galaxy samples selected by stellar-mass, evolv-
ods. Close pairs find galaxies before they merge, while ing luminosity, and constant co-moving number density
merger-induced morphological disturbances can appear at z < 1.5. We compare these results to the predicted
before, during, and after a galaxy merger. Moreover, major and minor galaxy merger rates. Throughout this
morphological studies use a variety of tracers to clas- work, we assume Ωm = 0.3, ΩΛ = 0.7, and H0 = 70 km
sify galaxies as mergers (such as G − M20 , asymmetry, s−1 Mpc−1 , except for close pair projected separations
tidal tails), which have differing sensitivities to different where h = H0 /100 km s−1 Mpc−1 .
merger properties such as mass ratio and gas fraction.
Thanks to new high-resolution hydrodynamical simula- 2. CALCULATING THE GALAXY MERGER RATE
tions of galaxy mergers which model realistic light pro- The volume-averaged galaxy merger rate Γmerg is de-
files including the effects of star-formation and dust (e.g. fined as the number of on-going merger events per unit
Jonsson et al 2006), it is now possible to estimate the co-moving volume, φmerg , divided by the time Tmerg for
timescales for detecting galaxy mergers with a variety of the merger to occur from the initial encounter to the final
close pair and morphological criteria (Lotz et al. 2008b; coalescence:
Lotz et al. 2010a, b). These simulations also span a φmerg
Γmerg = (1)
Tmerg
3. 3
Note that Tmerg , the time period between the time of ages. Such disturbances can be found through visual
the initial gravitational encounter and coalescence (from inspection by human classifiers, or by quantitative mea-
“not merging” to “merged”) is not well defined. surements of galaxy structures. Several different quanti-
More accurately, the number density of galaxies identi- tative methods are commonly used to measure galaxy
fied as galaxy mergers φmerg will depend on the average morphology and classify galaxy mergers. Rotational
timescale Tobs during which the merger can be observed asymmetry (A) picks out large-scale high surface bright-
given the method used to identify it, such that ness asymmetric structures (e.g. Abraham et al. 1994;
Conselice, Bershady, & Jangren 2000; Conselice 2003);
Tobs the combination of the Gini coefficient (G; Abraham et
φ′
merg = φmerg (2)
Tmerg al. 2003; Lotz et al. 2004) and second-order moment
of the brightest 20% of the light (M20 ) selects galaxies
A galaxy merger may be identified at discontinuous with multiple bright nuclei (Lotz et al. 2004). Both of
stages (as in the case of close pairs), therefore Tobs is these methods require high signal-to-noise and high spa-
the sum of the observability windows. tial resolution images generally only achievable with the
Thus, the galaxy merger rate Γ can be calculated from Hubble Space Telescope (HST ) at z > 0.3, as well as a
the observed number density of galaxy merger candidates training set to distinguish ‘normal’ galaxies from merger
φ′
merg as follows: candidates.
The merger fraction, fmerg , is the fraction of galaxies
φ′
merg Tmerg φ′
merg identified as mergers for a given galaxy sample, and is
Γmerg = = (3)
Tmerg Tobs Tobs often presented instead of the number density of observed
merger events. Thus φ′ merg depends on both the merger
The majority of past merger calculations have assumed fraction and the co-moving number density of galaxies in
Tobs to be a constant value ranging from ∼ 0.2 Gyr to 1 the selected sample, ngal :
Gyr, without considering the differences in methodology,
the effect of merger parameters, or redshift-dependent φ′
merg = fmerg ngal (4)
changes in the ability to detect mergers. In §4, we present
detailed calculations of Tobs as a function of redshift for For both morphologically-selected and close pair
different methods using theoretical models of galaxy in- merger candidates, a correction factor is applied to the
teractions and the evolving distribution of galaxy merger merger fraction to account for contamination from ob-
properties. jects that are not mergers. For morphological merger
Galaxy mergers can be directly identified as close selection, this correction is applied prior to calculating
galaxy pairs that have a high probability of merging fmerg based on visual inspection or a statistical correc-
within a short time. Galaxies with similar masses that tion. For close pairs, the fraction of pairs (fpair ) or the
lie within a hundred kpc of each other and have relative fraction of galaxies in a pair (Nc ∼ 2fpair ) is multiplied
velocities less than a few hundred km s−1 are likely to by this correction factor Cmerg , such that
merge within a few billion years. Individual close galaxy
pairs may be found by counting up the numbers of galax- Nc
fmerg = Cmerg fpair ∼ Cmerg (5)
ies with companions within a given projected separation 2
Rproj and within a relative velocity or redshift range. We will adopt Cmerg = 0.6 throughout this paper. Nu-
However, spectroscopic determination of the relative ve- merical simulations and empirical measurements suggest
locities of close pairs is observationally expensive, and that Cmerg is ∼ 0.4−1.0 for close pairs selected with
prone to incompleteness due to slit/fiber collisions and
biased detection of emission-line galaxies (e.g. Lin et al. projected separations < 20−30 kpc h−1 (Kitzbichler &
2004, 2008; de Ravel et al. 2009). Photometric red- White 2008, Patton & Atfield 2008; see Bundy et al. 2009
shifts are often used as a proxy for spectroscopic velocity for discussion). It is worth noting that close pair samples
separations (e.g. Kartaltepe et al. 2007, Bundy et al. are likely to be biased towards objects in groups or clus-
2009). However, even high precision photometric red- ters (Barton et al. 2007), although simulations suggest
δz that the majority of these objects will merge (Kitzbichler
shifts ( 1+z ∼ 0.02) are only accurate to 100-200 Mpc & White 2008).
along the line of sight. Therefore significant statistical Multiple authors have estimated the fractional merger
corrections for false pairs are required for photometrically rate ℜmerg instead of Γmerg where
selected pair studies (e.g. Kartaltepe et al. 2007). Fi-
nally, two-point angular correlation studies of large sam- fmerg Cmerg fpair
ples of galaxies have also been used to determine the ℜmerg = = (6)
Tobs Tobs
statistical excess of galaxies within a given dark matter
halo (e.g. Masjedi et al. 2006, Robaina et al. 2010). (e.g. L´pez-Sanjuan et al. 2009, Bundy et al. 2009,
o
These studies are less prone to line-of-sight projection Bridge, Carlberg, & Sullivan 2010, Conselice et al. 2009,
issues, but cannot identify individual merging systems. Jogee et al. 2009). In principle, the fractional merger
Morphological disturbances also indicate a recent or rate ℜmerg circumvents variation in ngal , which may
on-going merger. Galaxy mergers experience strong change from field to field with cosmic variance and is
gravitational tides, triggered star-formation, and the re- a factor of two or more lower at z ∼ 1 than z ∼ 0.2 for
distribution of their stars and gas into a single relaxed fixed stellar mass or passive luminosity evolution selec-
galaxy. This gravitational rearrangement results in mor- tion (Figure 2). Semi-analytic cosmological simulations
phological distortions – e.g. asymmetries, double nuclei, also have difficulty simultaneously reproducing the cor-
tidal tails – which are detectable in high-resolution im- rect galaxy number densities at all mass scales, but may
4. 4
be more robustly compared to observations for relative identifies merging systems for only a short period while
trends such as ℜmerg . double nuclei are evident (Lotz et al. 2008; Lotz et al.
Γmerg traces the number of merger events per co- 2010a, b).
moving volume above some mass/luminosity limit, By applying the G − M20 technique to high-resolution
while ℜmerg traces the number of merger events per HST Advanced Camera for Surveys V (F606W) and I
(bright/massive) galaxy. It is often implicitly assumed (F814W) images of the Extended Groth Strip (Davis et
that any evolution with redshift is not dependent on al. 2007), Lotz et al. (2008a) found fmerg = 10±2% for a
ngal (z) or the galaxy selection either because the par- sample of galaxies at 0.2 < z < 1.2. This work concluded
ent galaxy samples are selected in a uniform way or be- that the galaxy merger fraction and rate evolved weakly
cause the merger rate is not a strong function of galaxy over this redshift range: fmerg (z) ∝ (1 + z)0.23±1.03 . The
luminosity, mass, or number-density. However, several parent sample was selected to be brighter than 0.4L∗ (z),
B
studies have found that the fraction of mergers increases assuming passive luminosity evolution of galaxies and
significantly at fainter absolute magnitudes/lower stel- rest-frame B luminosity functions calculated for the Ex-
lar masses (Bridge et al. 2010; de Ravel et al. 2009; tended Groth Strip (MB < −18.94 − 1.3z; Faber et
Lin et al. 2004; Bundy et al 2005), while other studies al. 2007). Only rest-frame B morphologies were con-
have found increased merger fractions for more massive sidered in order to avoid biases associated with intrin-
galaxies (Bundy et al. 2009; Conselice et al. 2003). If sic morphological dependence on rest-frame wavelength.
the observed merger fraction depends upon luminosity The merger fractions were corrected for visually- and
or stellar mass, comparison of different observational es- spectroscopically-identified false mergers (∼ 20−30% of
timates of Γmerg or ℜmerg will require careful consider- initial merger candidates, similar to photometric redshift
ation of the parent galaxy sample selection criteria. We pair corrections). G − M20 identified merger candidates
will return to this issue in §3.5. with visually-identified artifacts or foreground stars were
removed from the sample. Also, G − M20 merger candi-
3. GALAXY MERGER OBSERVATIONS dates with multiple spectroscopic redshifts within a sin-
gle DEEP2/DEIMOS slit were identified and used to sta-
Despite the large number of galaxy merger studies,
tistically correct the resulting merger fractions (see Lotz
there is little consensus on the galaxy merger rate or
et al. 2008a for additional discussion.) The statistical
its evolution with redshift at z < 1.5. In part, this is
uncertainties associated with galaxies scattering from the
because many studies have compared the galaxy merger
main locus on normal galaxies to merger-like G−M20 val-
fraction fmerg (z) (Figure 1) or the galaxy merger rate for
ues are also included in the merger fraction error. The
galaxy merger samples selected with different approaches
fmerg and ngal values from Lotz et al. (2008a) are given
and/or with different parent sample criteria. In this sec-
in Table 2.
tion, we review the selection of galaxy mergers and their
For this paper, we have also computed fmerg (G − M20 )
parent samples for a variety of recent merger studies at
for a parent sample selected above a fixed stellar mass
z < 1.5. We will focus on works that identify merger
Mstar > 1010 M⊙ for the same HST observations of the
via quantitative morphology (G − M20 , A) or close pairs,
Extended Groth Strip. (Stellar masses were calculated
but briefly discuss results from other approaches. Almost
using Bruzual & Charlot 2003 spectral energy distribu-
all of these studies have calculated either the evolution
tions and a Chabrier 2003 initial mass function; see Salim
in the merger fraction (fmerg or fpair ), thereby ignoring
et al. 2009, 2007 for details). We estimated ngal us-
Tobs (z) , or the merger rate Γmerg or ℜmerg by assuming
ing the galaxy stellar mass functions v. redshift given
a constant Tobs with redshift.
by Ilbert et al. (2010) for the same redshift bins. We
note that these were derived using a different field (COS-
3.1. G − M20 -Selected Mergers MOS), but are consistent with the galaxy stellar mass
One quantitative morphological approach to identify- function computed for the Extended Groth Strip with
ing galaxy mergers in high-resolution images has been larger redshift bins (Bundy et al. 2006).
the G − M20 method. The Gini coefficient G is a mea-
surement of the relative distribution of flux values in the 3.2. Asymmetric Galaxies
pixels associated with a galaxy, such that uniform sur- Another commonly-used measure of morphological dis-
face brightness galaxies have low G and galaxies with disturbance is the rotational asymmetry, A. Asymmetry
very bright nuclei have high G (Abraham et al. 2003; measures the strength of residuals when a galaxys profile
Lotz et al. 2004). When G is combined with M20 , the is subtracted from its 180-degree rotated profile (Abra-
second-order moment of the brightest 20% of the light, ham et al. 1994; Conselice et al. 2000). Merger simula-
local spiral and elliptical galaxies follow a well-defined se- tions suggest that high A values may last for a longer pe-
quence and local mergers have higher G and and higher riod of time than G− M20 detected disturbances (Lotz et
M20 values (Lotz et al. 2004). Simulations of galaxy al. 2008b, 2010a,b). As we discuss in §4, A is highly sen-
mergers revealed that the G − M20 technique primarily sitive to gas fraction and, for local gas fractions, probes
identifies mergers during the first pass and final merger mergers with a different range of mass ratio than G−M20 .
(Lotz et al. 2008b) and is sensitive to mergers with bary- Early studies of small deep HST fields by Abraham
onic mass ratios between 1:1 - 1:10 (Lotz et al. 2010b). et al. (1996) and Conselice et al. (2003, 2005) showed
It has been noted by multiple authors that the merger strong evolution in the fraction of asymmetric galaxies
sample found by the G − M20 technique is incomplete at 0 < z < 3 with stellar masses > 1010 M⊙ . Subsequent
(Kampczyk et al. 2007; Scarlata et al. 2007; Kartaltepe studies based on larger HST surveys confirmed this ini-
et al. 2010). This is consistent with the results from tial result at 0 < z < 1.2 (Cassata et al. 2005, Conselice
galaxy merger simulations, which predict that G − M20 et al. 2009, L´pez- Sanjuan et al. 2009; but see Shi et
o
5. 5
Fig. 1.— Left: The galaxy merger fraction (fmerg ) or close pair fraction (fpair ) v. redshift for samples selected by stellar mass
(Mstar > 1010 M⊙ ). Right: Same, for samples selected with an evolving luminosity limit (see text). Large asterisks are G − M20 -selected
mergers, filled circles are for close pairs, and open diamonds are for asymmetric galaxies. For both stellar-mass and luminosity-selected
parent samples, fmerg and fpair vary by a factor of 10 for different merger studies.
al. 2009). For this paper, we consider the measurements values. Also, rather than visually inspecting the asym-
for three recent studies: Conselice et al. (2009), L´pez-
o metric merger candidates for false mergers, they employ
Sanjuan et al. (2009) and Shi et al. (2009). These studies a maximum-likelihood approach to determine how many
compute A in rest-frame B. Low signal-to-noise images normal galaxies are likely to be scattered to high asym-
and redshift-dependent surface-brightness dimming can metry values via large measurement errors and infer a
produce strong biases in the asymmetry measurements. high contamination rate (> 50%). Consequently, their
Each of these asymmetry studies makes different assump- merger fractions are much lower than Conselice et al.
tions about how to correct for these effects, and it is (2009), but follow similar evolutionary trends with red-
therefore perhaps not surprising that they draw different shift.
conclusions about fmerg (z). Shi et al. (2009) also examine the fraction of asymmet-
Conselice et al. (2009) computed asymmetry fractions ric galaxies in HST images of the GOODS fields. How-
for galaxies at 0.2 < z < 1.2 selected from the COS- ever, they select galaxies based on an evolving luminosity
MOS (Scoville et al. 2007 ), GEMS (Rix et al. 2004), cut (MB < −18.94−1.3z) and apply different corrections
GOODS (Giavalisco et al. 2004) and AEGIS (Davis et for the effects of sky noise and surface-brightness dim-
al. 2007) HST surveys. They find that the fraction of ming to the asymmetry measurements. Shi et al. (2009)
galaxies with stellar masses > 1010 M⊙ that have high find that the fraction of asymmetric galaxies has not
asymmetry increases from ∼ 4% at z = 0.2 to 14% at evolved strongly at z < 1 with fmerg (z) ∝ (1 + z)0.9±0.3 ,
z = 1.2, with fmerg (z) ∝ (1+z)2.3±0.4 . These asymmetry but find asymmetric fractions 2 − 3 times higher than
fractions are corrected for an assumed mean decrease in Conselice et al. (2009). Shi et al. (2009) note that in
asymmetry with redshift due to surface-brightness dim- addition to a redshift-dependent surface-brightness dim-
ming ( δA = −0.05 at z = 1). False merger candidates ming correction to the measured asymmetry values, cor-
scattered to high A values are visually identified and re- rection based on the signal-to-noise of the image may be
moved from the final asymmetric galaxy fractions, as are required. Extremely deep images (i.e. the Hubble Ultra
galaxies with high A but low clumpiness (S) values. Deep Field; Beckwith et al. 2006) can give asymmetry
L´pez-Sanjuan et al. (2009) also compute the frac-
o values 0.05 − 0.10 higher than lower signal-to-noise im-
tion of asymmetric galaxies in the GOODS fields se- ages of the same galaxy (Shi et al. 2009; Lotz et al.
lected from a parent sample at z < 1 with stellar masses 2006). Based on extensive simulations of this effect, Shi
> 1010 M⊙ . They find even stronger evolution in the et al. (2009) correct the observed fractions of asymmetric
merger fraction fmerg (z) ∝ (1 + z)5.4±0.4 . They ap- galaxies to that expected for high signal-to-noise obser-
ply a similar redshift-dependent correction for surface- vations in the local universe. No correction for falsely-
brightness dimming to the measured asymmetry values, identified merger candidates is applied, hence the Shi et
but correct their asymmetry values to the values ex- al. (2009) results may be considered an upper limit to the
pected to be observed at z = 1, rather than z = 0 as fraction of asymmetric galaxies while the L´pez-Sanjuan
o
Conselice et al. 2009 does. This results in lower fmerg et al. (2009) result may be considered a lower limit. It
6. 6
is also possible that the different signal-to-noise thresh- with the same data.) We will examine only the bright
olds for detecting asymmetry probe different processes sample results, as this selection (MB < −18.77 − 1.1z)
(clumpy star-formation, minor mergers, major mergers). is similar to the Lin et al. (2008) study. They adopt
a similar luminosity ratio for the paired galaxies (1:1 -
3.3. Close Pairs 1:4) to the Lin et al. 2008 study. They also compute the
Numerous studies have attempted to measure the pair fraction fpair at several different projected separa-
galaxy merger rate by counting the numbers of galax- tions. We use their values for Rproj < 100 kpc h−1 and
ies with close companions. However, these studies often δV ≤ 500 km s−1 because these have the smallest statis-
adopt different criteria for the projected separation of tical errors and give merger rates consistent with smaller
galaxies, the stellar mass (or luminosity) ratio of primary projected separation. The number density of galaxies
and companion galaxy, and the stellar mass (or luminos- was computed using the VVDS rest-frame B luminosity
ity) range of the parent sample. We can account for functions (Ilbert et al. 2005).
different projected separation criteria in our estimates of Kartaltepe et al. (2007) used photometric redshifts to
the merger rate by modifying the dynamical timescale. select objects at 0.2 < z < 1.2 with close companions
But without a priori knowledge of how the merger rate from deep K-band imaging of the COSMOS field. They
depends on mass ratio and mass, we cannot compare find that the pair fraction evolves strongly with redshift
merger studies where close pairs are selected with differ- fpair ∝ (1 + z)3.1±0.1 , in rough agreement with a later
ent mass ratios or from samples with different limiting study by Rawat et al. (2008) of J-band selected pairs
masses. Here we describe the different criteria adopted in the Chandra Deep Field South. To minimize the con-
by different studies, and how these criteria are likely to tamination from false pairs, they applied a stricter pro-
affect our derivation of the merger rate. We divide these jected separation criteria of 5 < Rproj < 20 kpc h−1 and
studies into those selected by stellar mass and those se- δzphot ≤ 0.05. They also selected pairs of galaxies where
lected by rest-frame luminosities. We calculate Tobs (z) both objects were brighter than a fixed absolute magni-
for each of the close pair selection criteria in §4, and as- tude MV = −19.8 which corresponds to L∗ at z = 0.
V
sume Cmerg = 0.6 for all samples when calculating the They correct the pair fractions for contamination rates
merger rates in §5. for each redshift bin (∼ 20−30%). The range of mass ra-
3.3.1. Luminosity-selected pairs tios is not well-defined, but > L∗ galaxies are rare, thus
the majority of pairs will have luminosity ratios between
Lin et al. (2008) find weak evolution in the pair frac-
1:1 and 1:4.
tion between 0 < z < 1.1, with fpair ∝ (1 + z)0.4±0.2 . Because their luminosity selection does not evolve with
This work reanalyzes paired galaxies from the Team Keck redshift, the Kartaltepe study probes a fainter parent
Treasury Redshift Survey (Wirth et al. 2004), DEEP2 population of galaxies at z ∼ 1 and brighter parent pop-
Galaxy Redshift Survey (Davis et al. 2004; Lin et al. ulation at z ∼ 0.2 relative to the de Ravel and Lin et
2004), CNOC2 Survey (Yee et al. 2000; Patton et al. al. spectroscopic pair studies and the Lotz et al. (2008a)
2002), and Millennium Galaxy Survey (Liske et al. 2003; G − M20 study. This makes it difficult to directly com-
de Propris et al. 2005) with a uniform selection criteria. pare the Kartaltepe et al. (2007) results to other stud-
They require that both objects in the pair have spectro- ies which adopt evolving luminosity selection. They do
scopic redshifts, relative velocities δV < 500 km s−1 , and consider the evolution of fpair when an evolving lumi-
projected separations 10 < Rproj < 30 kpc h−1 . In or- nosity cut MV < −19.8 + 1.0z is adopted, and find that
der to restrict their sample to major mergers, they select their conclusions about the evolution of fpair (z) do not
only galaxies and their companion within a limited rest- change. In this work, we will use the COSMOS pair
frame B luminosity range corresponding to a luminosity fractions selected with MV < −19.8 − 1.0z (J. Kartal-
ratio 1:1 - 1:4. Based on the evolution of the rest-frame tepe, private communication) to compare with the other
B galaxy luminosity function with redshift observed by merger studies. Because no published rest-frame V lu-
DEEP2 (Willmer et al. 2006; Faber et al. 2007), they minosity function is available for the COSMOS field, we
assume that the typical galaxy fades by 1.3MB per unit estimate ngal for MV < −19.8 − 1.0z from the Ilbert et
redshift, and therefore have a redshift-dependent lumi- al. (2005) rest-frame V -band luminosity function of the
nosity range −21 < MB + 1.3z < −19. An additional VVDS field. We will return to the issue of parent sample
complication is the correction for paired galaxies where selection and its effects on the evolution in the observed
a spectroscopic redshift is not obtained for one galaxy galaxy merger rate in §3.5 .
in the pair, either because of the spectroscopic survey Patton & Atfield (2008) select z ∼ 0.05 close pairs
sampling (not observed) or incompleteness (not measur- with spectroscopic redshifts from the Sloan Digital Sky
able). This correction is a function of color, magnitude, Survey. These are required to have δV < 500 km s−1 ,
and redshift, and therefore is not trivial to compute. We 5 < Rproj < 20 kpc h−1 , and rest-frame r-band lumi-
give Nc (uncorrected) values and completeness-corrected nosity ratios between 1:1 and 1:2. The parent sample is
pair fractions fpair from Lin et al. (2008) in Table 1. drawn from galaxies with −22 < Mr < −18. They derive
de Ravel et al. (2009) recently completed a similar a completeness-corrected value of Nc = 0.021 ± 0.001.
study, using galaxies with spectroscopic redshifts 0.5 < Patton & Atfield argue that between 50-80% of these
z < 0.9 from the VIRMOS VLT Deep Survey (VVDS). pairs are line-of-sight projections, higher than the 40%
They find that the evolution in the pair fraction depends (Cmerg ∼ 0.6) assumed here and in other works.
on the luminosity of the parent sample, with brighter
pairs showing less evolution (fpair ∝ (1 + z)1.5±0.7 ) than
fainter pairs (fpair ∝ (1 + z)4.73±2.01 ). (See also L´pez-
o 3.3.2. Stellar-mass selected pairs
Sanjuan et al. 2011 for an analysis of minor mergers
7. 7
In order to avoid assumptions about galaxy rest-frame
luminosity evolution, Bundy et al. (2009) select galaxy
pairs in the GOODS fields based on a fixed stellar mass
and find that the pair fraction does not evolve strongly at
z < 1.2, with fpair ∝ (1+z)1.6±1.6 . They compute stellar
masses by fitting the galaxies’ spectral energy distribu-
tions (0.4 − 2 µm) with Bruzual & Charlot (2003) stel-
lar population models, assuming a Chabrier (2003) stel-
lar initial mass function. The galaxy sample is selected
above a fixed stellar mass limit of Mstar ≥ 1010 M⊙ .
Close pairs are selected to be within 5 < Rproj < 20
kpc h−1 and with stellar mass ratios between 1:1 and
1:4. Bundy et al. (2009) includes paired galaxies
with both spectroscopic and photometric redshifts where
δz 2 < σz,primary + σz,satellite and the typical photomet-
2 2
ric redshift error σz < 0.08(1 + z). We estimate the
number density of GOODS galaxies with stellar masses
> 1010 M⊙ from Bundy et al. (2005).
In addition to their luminosity-based selection, de
Ravel et al. (2009) also select spectroscopic VVDS
galaxy pairs by stellar mass. The derived pair fraction
evolution depends on the stellar mass limit of the sample,
with more massive pairs showing weak evolution (fpair ∝
(1 + z)2.04±1.65), consistent with the Bundy et al. (2009)
results. Their stellar masses are computed with similar
population synthesis models and multi-wavelength pho-
tometric data to the Bundy et al. (2009). de Ravel
et al. (2009) assumes a modest evolution of stellar mass
and selects a parent sample with an evolving stellar mass
limit log[Mstar /1010 M⊙ ] > 0.187z. As for the luminosity Fig. 2.— Top: Number of galaxies per co-moving unit volume
selected sample, we use de Ravel et al. ’s spectroscopic ngal v. redshift for the evolving luminosity selection calculated
pairs with Rproj < 100 kpc h−1 and δV ≤500 km s−1 . from Ilbert et al. 2005 (VVDS V -band selected, red diamonds;
Galaxy pairs are required to have stellar mass ratio be- VVDS B-band selected blue diamonds) and Faber et al. 2007
(DEEP2 B-band selected, green asterisks); Bottom: ngal (z) for
tween 1:1 and 1:4. We estimate the number density of
galaxies selected with Mstar > 1010 M⊙ from Bell et al. 2003
galaxies from the Pozzetti et al. (2007) K-selected stellar (2MASS, black square), Bundy et al. 2005 (GOODS, black filled
mass functions computed for the VVDS. circles), Ilbert et al. 2009 (black diamonds), Pozzetti et al. 2007
(VVDS, open circles), and Bundy et al 2006 (DEEP2, black aster-
3.4. Visually Classified Mergers isks).
The visual classification of morphologically disturbed
galaxies has a long history (e.g. Hubble 1926), and
Bridge et al. (2010) study is based upon very deep
has increased in scale and sophistication. Several recent
ground-based images from the CFHT Legacy Survey,
studies have estimated the galaxy merger fraction and
therefore has worse spatial resolution than HST -based
rate using large samples of visually-classified mergers at
studies but reaches comparable limiting surface bright-
z < 1.5, including the Galaxy Zoo project (Darg et al.
nesses over a much wider area. They identify merg-
2010; also Jogee et al. 2009; Bridge et al. 2010; Kartal-
ing and interacting galaxies via extended tidal tails and
tepe et al. 2010). However, studies of the evolution of
bridges for a sample of ∼ 27,000 galaxies with i < 22
visually-classified mergers also reach contradictory con-
Vega mag. They find the merger fraction of galaxies
clusions.
with i < 22 and stellar mass > 3 × 109 M⊙ increases
Jogee et al. (2009) visually classified galaxies with stel-
rapidly from ∼ 4% at z ∼ 0.4 to ∼ 19% at z ∼ 1, with
lar masses > 2.5 × 1010M⊙ at 0.24 < z < 0.80 in V -band an evolution ∝ (1 + z)2.25±0.24 .
HST ACS images from the GEMS survey. They iden- We will not attempt to incorporate these results into
tify mergers as objects with asymmetries, shells, double our study here. Visual classification of galaxy mergers
nuclei, or tidal tails but exclude obvious close or inter- is qualitative by nature, and the exact merger identi-
acting pairs. They find that ∼ 9% of the sample were fication criteria applied for each study depends on the
visually-disturbed with little evolution between z ∼ 0.2 human classifiers as well as the spatial resolution and
and z ∼ 0.8. Between 1-4% of the sample are classified as depth of the imaging data. Therefore visual classifica-
major mergers and 4-8% are classified as minor mergers. tion observability timescales are unique to each study
They conclude that while most massive galaxies have un- and less straightforward to calculate than for the quanti-
dergone a major or minor merger over the past 7 Gyr, tative and easily reproducible (but possibly less sensitive;
visually-disturbed galaxies account for only 30% of the Kartaltepe et al. 2010) methods that we use in this work.
global star-formation at those epochs.
Bridge et al. (2010) also visually identify galaxy merg- 3.5. The Importance of Sample Selection
ers at z < 1, but reach fairly different conclusions. The
8. 8
Often the merger fraction fmerg is measured over a from merger-induced starburst. This effect could bias
range of redshifts where the galaxy mass and luminosity luminosity-selected samples similarly, and give different
functions change significantly (e.g. Faber et al. 2007; merger rates than samples selected by stellar mass. How-
Ilbert et al. 2010). If the galaxy merger rate is a func- ever, merger simulations suggest that luminosity bright-
tion of stellar mass or luminosity (e.g. Lin et al. 2004; ening in the rest-frame optical is mitigated by dust ob-
Bundy et al. 2005; de Ravel et al. 2009; Bridge et al. scuration of the starbursts (Jonsson et al. 2006 ). In
2010), then one must be careful about comparing studies §5, we find little difference between the merger rates for
with different parent selection criteria. In an attempt to luminosity and stellar-mass selected samples, implying
sample similar galaxies over a wide redshift range, the that luminosity brightening does not introduce significa-
parent galaxy sample may be selected to be more mas- tion biases to the merger rate.
sive than a fixed stellar mass (e.g. Bundy et al. 2009, Finally, it is also important to note that the standard
Conselice et al. 2009, de Ravel et al. 2009, Jogee et selection of galaxy sample for galaxy merger studies (e.g.
al. 2009, Bridge et al. 2010) or by adopting an evolving above a fixed stellar mass) is not well matched to the
luminosity selection based on a passive luminosity evo- way that dark matter halos are selected from simulations
lution (PLE) model in which galaxies of a fixed stellar for dark matter halo merger studies (e.g. above a fixed
mass are fainter at lower redshift due to passive aging of halo mass). We know that typical blue galaxies are not
their stellar populations (e.g. Lotz et al. 2008, Lin et al. evolving purely passively, but form a significant number
2008, de Ravel et al. 2009, Shi et al. 2009). of new stars at z < 1 (e.g. Lilly et al. 1996 ; Noeske
We compare the number densities of galaxies selected et al. 2007). Therefore the relationship between stellar
with passive luminosity evolution assumptions (Fig. 2, mass and host dark matter halo mass also evolves with
upper panel) and with a fixed stellar mass (Fig. 2, lower redshift (Zheng, Coil, & Zehavi 2007; Conroy & Wechsler
panel). The number densities for the PLE cuts MB < 2009; Moster et al. 2010). Given that the co-moving
−18.94 − 1.3z and MV < −19.8 − 1.0z and the fixed number density of galaxies with stellar masses > 1010 M⊙
stellar mass cut Mstar ≥ 1010 M⊙ are similar at z < 1, or selected based on PLE assumptions is as much as a
suggesting that the evolving luminosity selections of Lin factor of four lower at z ∼ 1.5 than z ∼ 0, it is clear that
et al. 2008, Lotz et al. 2008, Kartaltepe et al 2007, neither of these galaxy samples select the progenitor and
Shi et al. 2009, and the fixed stellar-mass selection of descendant galaxies across the range of redshifts.
Bundy et al. 2009, Conselice et al. 2009, de Ravel et al. Galaxy samples selected with a constant number den-
2009, and L´pez-Sanjuan et al. 2009 probe similar galaxy
o sity may do a better job of matching descendant-
populations. However, the number density of galaxies progenitor galaxies over a range of redshifts (van
selected by the de Ravel et al. (2009) MB < −18.77 − Dokkum et al. 2010; Papovich et al. 2010), and matching
1.1z cut is higher at higher z than the other selections. galaxies to constant mass halos (at Mhalo ∼ 1011−12 M⊙ ;
When a fixed luminosity (‘no evolution’) cut is adopted, Zheng et al. 2007). This is not a perfect selection crite-
as in Kartaltepe et al. 2007 and Rawat et al. 2008, rion, as it assumes galaxy mergers do not destroy signif-
the differences in the parent sample at z ∼ 1 are even icant numbers of galaxies and that stochasticity in the
more significant, which could explain the difference in luminosity/mass evolution of galaxies does not strongly
the derived evolution of the mergers. We will return to affect the sample selection. In §5.4, we present galaxy
this issue in §5.4. merger rates for parent galaxy samples selected with a
An additional complication is that merging galaxies roughly constant number density at z < 1.5, and com-
increase in stellar mass and luminosity as they merge, pare these to the rates derived for the fixed stellar mass
as well as undergo shorter-lived starbursts. Therefore and PLE samples.
late-stage mergers and merger remnants will be more
massive and luminous than their progenitors. Mor-
phological disturbances are often late stage mergers 4. MERGER OBSERVABILITY TIMESCALES
whose nuclei are counted as a single objects, while Knowledge of the average merger observability
close pairs are early-stage mergers (e.g. Lotz et al. timescale Tobs is crucial for calculating the galaxy
2008). Thus morphologically-disturbed mergers at a merger rate. This timescale will depend upon the method
given mass/luminosity are drawn from a less-massive used to select galaxy mergers, as close pairs pick out dif-
progenitor population than close pairs selected at the ferent merger stages from objects with disturbed mor-
same mass/luminosity limit. In the worst case of equal- phologies. For a given merger system, the individual
mass mergers, the morphologically-disturbed mergers observability timescale will also depend on the param-
will have progenitors 50% less massive and up to a mag- eters of the merger, such as the initial galaxy masses,
nitude fainter than close pairs. The inferred merger morphologies and gas fractions, the merger mass ra-
rate from morphologically-disturbed objects could be bi- tio, the orbit, and the relative orientation of the merg-
ased to higher values than the close-pair merger rate if ing galaxies. The initial properties of the merging sys-
the number density of merger progenitors increases with tem are difficult to reliably recover from observations
lower stellar mass. However, as we discuss in the next of an individual merging system, particularly after the
section, both G−M20 and A are sensitive to minor merg- merger has progressed to late stages. Moreover, the ob-
ers as well as major mergers. Therefore the difference served merger population is a mixture of merger events
between the typical (primary) progenitor’s and the rem- with a broad distribution of parameters which may be
nant’s stellar mass is less than 25% for morphologically- evolving with redshift. Therefore the mean observability
selected samples, assuming a typical merger ratio 1:4. timescale Tobs given in Eqn. 3 should be treated as a
A related issue is short-lived luminosity brightening of cosmologically-averaged observability timescale weighted
both close pairs and morphologically-disturbed galaxies by the distribution of merger parameters at each epoch.
9. 9
Because the observations of a given galaxy merger tion and time-step were run through the Monte-Carlo
are essentially an instantaneous snapshot at a particu- radiative transfer code SUNRISE to simulate realistic
lar merger stage, we cannot know how long that merger ultraviolet-optical-infrared images, including the effects
event will exhibit disturbed morphology or other obvious of star-formation, dust, and viewing angle. The details of
merger indicators. And, depending on the merger stage the SUNRISE code and the predicted effects of dust on
and viewing angle, it can be difficult to determine its ini- the integrated spectral energy distributions are described
tial conditions. Therefore, the best way to determine how in Jonsson 2006, Jonsson, Groves, & Cox 2010. In Lotz
the morphology and projected separation of a merger et al. (2008b, 2010a, 2010b), the simulated broad-band
progresses and how this depends on the merger condi- SDSS g images were used to calculate the time during
tions is by studying a large number of high-resolution nu- which each individual galaxy merger simulation would be
merical simulations of individual mergers where the ini- counted as a merger candidate by the quantitative mor-
tial merger conditions are systematically explored. Also, phology G − M20 and asymmetry methods. We also cal-
we do not yet have good observational constraints on the culate close pair timescales for several different projected
distribution and evolution of galaxy merger parameters separation criteria (5 < Rproj < 20, 10 < Rproj < 30,
such as gas fraction or mass ratio. Thus we will use 10 < Rproj < 50, and 10 < Rproj < 100 kpc h−1 ); all
theoretical predictions from three different global galaxy simulations have merging galaxies with relative veloci-
evolution models of the distribution of galaxy proper- ties less than 500 km s−1 . Note that for the close pair
ties and their evolution to constrain the distribution of and G − M20 methods, the observability windows are not
merger parameters with redshift. contiguous in time and therefore the observability time
In this section, we summarize the results of recent cal- is the sum of the times when the merger is selected, e.g.
culations of the individual observability timescales from as a close pair before and after the first pass.
a large suite of high-resolution disk-disk galaxy merger We found that, for a given method, Tobs depended most
simulations for the G − M20 , A, and close pair methods. on the mass ratio of the merger (Lotz et al. 2010a)
Then we examine the predicted distributions of two key and on the gas fractions of initial galaxies (Lotz et al.
merger parameters – baryonic gas fraction and mass ratio 2010b). Orbital parameters (eccentricity, impact pa-
– and their evolution with redshift from three different rameters) and total mass of the initial galaxies had lit-
cosmological-scale galaxy evolution models (Somerville tle effect on Tobs for morphological disturbances. We
et al. 2008; Croton et al. 2006; Stewart et al 2009b). will refer to the baryonic mass ratio of the merger
Finally, we use these distributions to weight the individ- Msatellite /Mprimary , where major mergers have baryonic
ual observability timescales and derive the average ob- mass ratios between 1:1 and 1:4 and minor mergers have
servability timescales as a function of redshift Tobs (z) baryonic mass ratios less than 1:4. We have chosen to
for each galaxy evolution model and approach to detect- characterize the merger mass ratio by the initial bary-
ing galaxy mergers. We compare the predictions of the onic mass ratio rather than total or stellar mass ratio
different galaxy evolution models and discuss the uncer- as a compromise between observational and theoretical
tainties associated with Tobs (z) . The derived Tobs (z) limitations. The total mass ratio of mergers is difficult
are applied to the observed merger fractions described in to estimate (or even define) observationally because this
§3 to compute merger rates in §5. requires a dynamical estimate of mass associated with
4.1. Merger Timescales from Dusty Interacting Galaxy each interacting galaxy. On the other hand, the stellar
GADGET/SUNRISE Simulations masses and stellar mass ratios predicted by cosmologi-
cal galaxy evolution models can be quite sensitive to the
The individual merger observability timescales Tobs uncertain physics of star formation and feedback (see,
were calculated for a large suite of high-resolution N- for example, Benson et al. 2003). As a result, a galaxy
body/hydrodynamical galaxy merger simulations in Lotz merger with a total mass ratio of 1:3 may correspond to
et al. 2008b, 2010a, b (also known as DIGGSS, Dusty a stellar mass ratio of anywhere from 1:10 to 1:2 (Stewart
Interacting Galaxy GADGET/SUNRISE Simulations; 2009a). While baryonic mass ratios suffer the same com-
see http://archive.stsci.edu/prepds/diggss for simulation plication, one may expect the ratio of baryonic-to-total
images and morphology measurements). Each GAD- mass to vary less strongly with redshift, since higher gas
GET simulation tracks the merger of two disk galaxies fractions at earlier times help offset the proportionately
within a box of ∼ (200 kpc)3 with a spatial resolution smaller stellar-to-total masses of ∼ L∗ galaxies.
∼ 100 pc and a particle mass ∼ 105 M⊙ , over a several Likewise, we characterize the merger gas fraction as
billion year period in ∼ 50 Myr timesteps. Cold gas is the initial fraction of the baryons in cold gas. We define
converted into stars assuming the Kennicutt-Schmidt re- the initial baryonic gas fraction fgas of the merger as
lation (Kennicutt 1998). The effects of feedback from
supernovae and stellar winds using a sub-resolution ef- (Mgas,1 + Mgas,2 )
fective equation of state model is included, but feedback fgas = (7)
(Mgas,1 + Mgas,2 + Mstar,1 + Mstar,2 )
from active galactic nuclei (e.g. Di Matteo et al. 2008)
is not. The full suite of simulations span a range of ini- This is the average gas fraction of the system prior
tial galaxy masses and mass ratios, gas fractions, and to the merger, when the galaxies first encounter each
orientations and orbital parameters and are described in other. Because these simulations do not accrete addi-
detail in Cox et al. 2006, 2008. The DIGGSS simulations tional gas and gas is converted into stars, the baryonic
do not include any fgas < 0.2 simulations or spheroidal gas fraction at the final merger is lower than the ini-
merger simulations. tial value. For this work, we use a sub-set of DIGGSS
The GADGET predictions for the ages, metallicities, which have parabolic orbits, tilted prograde-prograde ori-
and 3-D distribution of stars and gas for each simula- entations and initial galaxy parameters tuned to match
10. 10
Fig. 3.— The observability timescales Tobs for individual prograde-prograde disk-disk galaxy merger simulations v. baryonic mass ratio
(Mprimary /Msatellite ) from Lotz et al. 2010a,b. The simulations were run with three different baryonic gas fractions fgas = 0.2 (red
diamonds: G3G3Pt, G3G2Pt, G3G1Pt, G3G0Pt, G2G2Pt, G2G1Pt, G2G0Pt), 0.4 (green squares: G3gf1G3gf1, G3gf1G2, G3gf1G1) , and
0.5 (blue asterisks: G3gf2G3gf2, G3gf2G2, G3gf2G1).
Fig. 4.— The observability timescales Tobs for individual prograde-prograde disk-disk galaxy merger simulations v. baryonic gas fraction
fgas from Lotz et al. 2010a,b. Here, the points designate three different baryonic mass ratios Msatellite /Mprimary 1:1(red diamonds:
G3G3Pt, G3gf1G3gf1, G3gf2G3gf2), 3:1 (green squares: G3G2Pt, G3gf1G2, G3gf2G2), and 9:1 (blue asterisks: G3G1, G3gf1G1, G3gf2G1).
SDSS galaxies, but span a range of galaxy mass (Mstar ∼ ability timescales Tobs for G − M20 , A, and close pairs
1 × 109 − 5 × 1010 M⊙ ) and baryonic mass ratios (1:1 - with 10 < Rproj < 30 kpc h−1 depend on baryonic mass
1:39): G3G3Pt, G3G2Pt, G3G1Pt, G3G0Pt, G2G2Pt, ratio and fgas . Note that these are averaged over 11
G2G1Pt, G2G0Pt. These simulations have gas fractions different viewing angles (see Lotz et al. 2008b). The
tuned to local galaxies, with the initial baryonic gas frac- error-bars on Tobs in Figures 3 and 4 are the standard
tions fgas ∼ 0.2. We also use the subset of DIGGSS deviation with viewing angle of Tobs for each simulation.
which have the same parabolic orbits, tilted prograde- G−M20 and close pair observability timescales are largely
prograde orientations, and mass ratios but higher gas independent of gas fraction but are sensitive to mass ra-
fractions (fgas ∼ 0.4, 0.5) : G3gf1G3gf1, G3gf1G2, tio, while asymmetry A is sensitive to both gas fraction
G3gf1G1, G3gf2G3gf2, G3gf2G2, G3gf2G1. See Lotz et and mass ratio. G − M20 detects mergers with baryonic
al. (2010a, b) and Cox et al. (2008) for more details of mass ratios between 1:1 and at least 1:10, and does not
these simulations. show a strong correlation of Tobs with baryonic mass ra-
In Figures 3 and 4, we show how the individual observ- tio above 1:10. Mergers are not found with the G − M20
11. 11
criteria for the two simulations with baryonic mass ratios 4.2. Predicted Distribution of Merger Properties
less than 1:10 (G2G0Pt, G3G0Pt), therefore we conclude We calculate the cosmologically-averaged observability
that Tobs ∼ 0 for mergers at these mass ratios. The sim- timescale Tobs (z) for each method for finding mergers
ulations imply that G − M20 detects mergers at the stage from the individual Tobs given in Figures 3 and 4 and as-
when two bright nuclei are enclosed in a common enve- sumptions about the distribution of galaxy merger mass
lope, and that satellites with masses greater than a tenth ratios and fgas as follows:
of the primary galaxies are bright enough to be detected
(see Lotz et al. 2008a, Lotz et al 2010a for discussion).
Dynamical friction and the effects of projection largely Tobs (z) = wi,j (z) × Ti,j (8)
drive the close pair timescales, hence their insensitivity to i,j
fgas . The close pair timescales are correlated with mass
and mass ratio such that minor mergers appear as close where wi,j (z) is the fraction of mergers at redshift z with
pairs for a longer period of time than major mergers, baryonic mass ratio i and baryonic gas fraction j, and Ti,j
as do lower-mass major mergers (Table 5 in Lotz et al. is the observability timescale for a merger with baryonic
2010a). mass ratio i and baryonic gas fraction j. (Although the
The behavior of A observability timescales is more timescales do not change much with progenitor mass, in
complicated because of their dependence on both mass practice we also sum over two bins in primary progenitor
ratio and gas fraction. The simulations suggest that baryonic mass based on the input simulation primary
high asymmetries are the result of large-scale distur- galaxy masses of 2.0 × 1010 , 6.2 × 1010 M⊙ ).
bances, including bright star-forming tidal tails and dust We currently have very little empirical knowledge of
lanes, thus asymmetry is higher for more gas-rich mergers the distribution of baryonic mass ratios or gas fractions
which are more likely to form strong tidal features (see for merging galaxies. Therefore we assume the rela-
Lotz et al. 2010b). At gas fractions expected for typical tive distributions of mass ratios, gas fractions, and mass
local disk galaxies (fgas ∼ 0.2), A detects primarily ma- as a function of redshift predicted by three different
jor mergers with baryonic mass ratios between 1:1 and cosmological-scale galaxy evolution models: Somerville
1:4, and has Tobs ∼ 0 for more minor mergers with bary- et al. 2008 [S08]; Croton et al. 2006 [C06]; and Stewart
onic mass ratios 1:10 (Figure 3, red points, center panel). et al. 2009b [St09]. Note that for the timescale calcu-
But for simulations with fgas ≥ 0.4, A detects both 1:1- lations, we make use of only the relative distributions of
1:4 major mergers and 1:10 minor mergers (Figure 3, merger properties from these simulations and therefore
center panel, green and blue points). The observability are independent of their predicted galaxy merger rates.
timescale for A is strongly correlated with fgas (Figure Each of these models starts with a mass distribution
4, center panel), with high gas fraction mergers showing and merger history (merger tree) for the dark matter
high asymmetries for significantly longer periods of time central and sub-halos. S08 uses an analytically-derived
than low gas fraction mergers. Therefore knowledge of dark matter merger tree (Somerville & Kolatt 1999) with
both the merger gas fraction and mass ratio distributions a Sheth & Tormen (1999) dark matter halo mass func-
is required to estimate Tobs for asymmetry. tion; the C06 and St09 merger trees are derived from
For this work, we will ignore the effect of the orbital pa- N-body simulations (the Millennium Simulation from
rameters and relative orientations of the merging galax- Springel et al. 2005 and an Adaptive Refinement Tree
ies on the observability timescales. All of the simulation N-body simulation from A. Klypin described in Stew-
timescales adopted here are for the G-series prograde- art et al. 2008, respectively). These dark matter halos
prograde orientations, with e = 0.95 and pericentric dis- are populated with galaxies, either via a semi-analytic
tances ∼ 0.01 − 0.05 times the viral radii of the pro- approach that adopts physical prescriptions for galaxy
genitors. In Lotz et al. (2008b) and (2010a), we found formation (C06, S08) or via a semi-empirical approach
that different orbits and orientations had a weak effect that matches the expected clustering and abundances
on the observability timescales for G − M20 and A. How- of dark matter halos to the observed galaxy popula-
ever, galaxies merging on circular orbits or with large tion (St09). In the semi-analytical models, gas cools
impact parameters were identified as close pairs for 15- onto the dark matter halos and is converted into stars
40% longer than parabolic orbits with smaller impact assuming the Kennicutt-Schmidt law (Kennicutt et al.
parameters, depending on the projected separation re- 1998). The semi-analytic models also adopt similar pre-
quirements. Large-scale numerical simulations find that scriptions for feedback from supernovae and both high-
the majority of dark-matter halo mergers are parabolic luminosity quasars and lower luminosity active galactic
as opposed to circular (Khochfar & Burkert 2006; Ben- nuclei. The S08 models are tuned to match the gas frac-
son 2005; Wetzel 2010), with only modest evolution in tions of local galaxies. By contrast, the semi-empirical
the typical orbital eccentricity out to z ∼ 1.5 (Wetzel model of St09 assigns the stellar masses and gas masses
2010). Our adopted e is consistent with the mean value of galaxies based on observationally-derived correlations,
for dark matter halo -satellite mergers from Wetzel (2010; with stellar masses following the abundance matching
e ∼ 0.85), while our pericentric distances are smaller technique of Conroy & Wechsler (2009), and cold gas
than mean (∼ 0.13 Rvir ). If true pericentric distances fractions following an empirical fit to the results of Mc-
are significantly larger than our assumptions here, our Gaugh (2005) at z ∼ 0 and Erb et al. (2006) at z ∼ 2
simulations imply that this could systematically increase (see Stewart et al. 2009a,b for additional details).
the merger timescales (and decrease merger rates) by up For each galaxy evolution model, we select galaxy
to 10-40% for close pairs and a factor of 3-4 for G − M20 mergers with total stellar masses (Mstar,1 + Mstar,2 )
selected major mergers (Lotz et al. 2008b). ≥ 1010 M⊙ , baryonic mass ratios ≥ 1:10, and 0 < z < 1.5.
The galaxy stellar mass functions at z < 1.5 from all of
12. 12
Fig. 5.— The normalized distribution of baryonic mass ratios for galaxy mergers selected from the Somerville et al. 2008 (black histogram),
Croton et al. 2006 (blue), and Stewart et al. 2009b (red) models. The mergers were selected to be in two redshift bins ( 0.2 < z < 0.6;
solid lines; 0.6 < z < 1.4 dashed lines), with total stellar masses (Mstar,1 + Mstar,2 ) > 1010 M⊙ ), and baryonic mass ratios ≥ 1:10. There
is good agreement between all three models, and little evolution in the distribution with redshift. (Baryonic mass ratios greater than unity
arise when the more massive galaxy has less baryons but more dark matter than its companion).
z < 1.5 and for galaxies with masses above ∼ 1010 M⊙
(see Fontanot et al. 2009, Kitzbichler & White 2007).
We compare the distribution of baryonic mass ratios
and gas fractions for the three models in Figures 5 and 6.
In Figure 5, we find that the S08, St09, and C06 models
predict similar baryonic mass ratio distributions despite
the different derivations of the dark matter halo mass
function, merger tree, and baryonic masses. We find a
small increase in the relative fraction of minor mergers
( Msat /Mprimary < 0.25) at higher redshifts for the S08
and St09 models, while C06 predicts no change in the
mass ratio distribution.
However, the models make different predictions about
the merger gas properties (Figure 6). In all the models,
fgas increases significantly with redshift such that the
mean baryonic gas fraction of the mergers roughly dou-
bles from z = 0 to z = 1. The mean fgas values for all 1:1
- 1:10 mergers predicted by the S08 and St09 models are
in good agreement. On the other hand, the mean fgas in
the C06 models is much lower at all redshifts. Both S08
Fig. 6.— The predicted mean baryonic gas fraction fgas v. and C06 predict that major mergers with baryonic mass
redshift for 1:1 - 1:10 baryonic mass ratio mergers (solid) and ma- ratios less than 1:4 have lower mean gas fractions than
jor 1:1-1:4 baryonic mass ratio mergers (dashed line). All models
predict strong evolution in fgas with redshift, but the Somerville minor mergers. On the other hand, St09 predict that
et al. 2008 (black lines) and Stewart et al. 2009b (red lines) merg- major mergers have fgas similar to the overall merger
ers have higher fgas than the Croton et al. 2006 models (blue population.
lines). For comparison, we have plotted the observed fgas at z > 1 In Figure 6, we also compare the predicted mean gas
for the Tacconi et al. 2010 (squares) and Daddi et al 2010 (as-
terisks) samples, and the average fgas and standard deviation for fractions to recent measurements of the cold gas fraction
the McGaugh 2005 sample of local Mstar > 1010 M⊙ disk galaxies for massive disk galaxies at z ∼ 1 − 1.5 from Tacconi
(diamond). et al. (2010) and Daddi et al. (2010). Although these
pioneering studies have small and biased samples, they
indicate that the typical baryonic gas fractions of massive
the models are in good agreement with each other. St09 disk galaxies are several times higher at z ∼ 1 − 1.5
adopts the sub-halo mass - stellar mass matching rela- than locally (e.g McGaugh et al. 2005), in reasonable
tionship from Conroy & Wechsler (2009), based upon the agreement with the Somerville et al. (2008) and Stewart
observed stellar mass functions at 0 < z < 2 (Bell et al. et al. (2009b) models.
2003, Panter et al. 2007, Dory et al. 2005, Borsch et al. For each model, we compute the weight wi,j (z) for
2006, P´rez-Gonz´lez et al. 2008, Fontana et al. 2006).
e a three bins in baryonic mass ratio and four bins in fgas
The S08 and C06 predictions for the galaxy stellar mass for δz = 0.2 redshift bins from z = 0 to z = 1.6, nor-
functions (assuming a Chabrier initial mass function and malized to all merging systems in each redshift bin with
Bruzual & Charlot 2003 SEDs) are found to be in good (Mstar,1 + Mstar,2 ) ≥ 1010 M⊙ and baryonic mass ratios
agreement with these same observed measurements at
13. 13
Fig. 7.— The fraction of mergers wi,j with baryonic mass ratio i and baryonic gas fraction j v. redshift from the Somerville et al. (2008)
models (top panels), the Croton et al. (2006) models (middle panels), and Stewart et al. (2009b) models (bottom panels). The weights
have been calculated for three baryonic mass ratio bins ( 1:1-1:2, left panels; 1:2-1:6, center panels; 1:6-1:10, right panels) and four baryonic
gas fraction bins (fgas < 0.1, red lines; 0.1 < fgas < 0.3, orange lines; 0.3 < fgas < 0.45, green lines; fgas > 0.45, blue lines.) The black
lines in each panel show the fraction of mergers of all gas fractions for that panel’s mass ratio bin.
≥ 1:10 (Figure 7). The spacing of these bins is deter- models predict that lower gas fraction mergers dominate
mined by the simulation parameter space for the SDSS- the z = 0 merger population (red and yellow lines). How-
motivated galaxy mergers: 1:1 - 1:2 (major), 1:2-1:6 (in- ever, S08 predicts strong evolution in the gas properties
termediate), and 1:6-1:10 (minor) baryonic mass ratio of mergers, such that by z ≥ 1, the merger population
bins; and 0.0-0.1, 0.1-0.3, 0.3-0.45, and 0.45-1.0 fgas bins. is dominated by high gas fraction (fgas > 0.45) inter-
The three baryonic mass ratio bins are plotted in sepa- mediate and minor mergers (blue lines, middle and right
rate panels (major:left, intermediate:center, and minor: panels). Because C06 predicts low mean fgas at z < 1.5,
right). The four fgas bins are plotted with different color they predict that the lowest gas fraction mergers con-
lines in each mass ratio panel. The resulting weights are tinue to dominate at z > 1 (red lines). Like S08, the
shown for the three galaxy evolution models (S08: top, St09 model predicts higher mean fgas at z ≥ 1. But
C06: middle, St09: bottom). St09 has a different distribution of fgas at a given red-
For all three models, the baryonic mass ratio distribu- shift. St09 has fewer very gas-rich mergers and more
tion changes little with redshift, and intermediate mass very gas-poor mergers at z ∼ 1, and predicts that most
ratio mergers (1:2 - 1:6; middle panels) are 50% of the mergers at 0 < z < 1.5 have intermediate fgas ( ∼ 0.2)
total (1:1 -1:10) merger population at all redshifts. The in all mass ratio bins (yellow lines). As we will discuss
predicted weights wi,j (z) vary most in the predicted dis- in the next section, the relative frequency of gas-rich in-
tribution and evolution of merger gas fractions. All three termediate and minor mergers at z ≥ 1 has important
14. 14
implications for the interpretation of asymmetric galax- mass ratios is not predicted to evolve with redshift. The
ies at this epoch. timescales for close pairs selected with baryonic mass ra-
tios between 1:1 and 1:4 are ∼ 0.33 Gyr for 5 < Rproj <
4.3. Average Merger Timescales Tobs (z) 20 kpc h−1 , and ∼ 0.63 Gyr for 10 < Rproj < 30 kpc
h−1 (Table 1; Figure 8).
We compute Tobs (z) for G − M20 , asymmetry, and
We find that Tobs (z) for G − M20 is also very similar
close pairs with 5 < Rproj < 20, and 10 < Rproj <
for all three model weights, and does not evolve with
30 kpc h−1 , using Eqn. 8, the Ti,j computed from redshift. This is not surprising as G − M20 Tobs does not
the GADGET-SUNRISE simulations as a function of correlate with fgas nor baryonic mass ratio between 1:1
baryonic mass ratio and gas fraction (Figures 1 and and 1:10. Therefore, the G−M20 observability timescales
2), and the relative weights wi,j computed from each are not sensitive to the underlying assumptions about the
of the cosmological galaxy evolution models. These distribution of merger gas fractions or mass ratios. For
cosmologically-weighted timescales Tobs (z) are plotted mergers with stellar masses ≥ 1010 M⊙ at 0 < z < 1.5,
as a function of redshift for the three different models for Tobs (z) is ∼ 0.21 Gyr for G − M20 (Table 2; Figure 9
each method for finding mergers (Figures 8 and 9). upper panel).
The relative weights wi,j are adjusted for the mass ra- The cosmologically-averaged timescales for asymmetry
tio sensitivity for each technique, so that the resulting are the most sensitive to the assumptions about the joint
merger rates are not extrapolated to mass ratios below distribution of merger gas fractions and mass ratios (Fig-
what is detected. Therefore the weights for G − M20 and ure 9). The uncertainty in the asymmetry timescales of
A are normalized for all mergers with baryonic mass ra- fgas < 0.1 gas-poor mergers changes Tobs (A) by less
tios between 1:1 and 1:10, and assume no mergers with than 0.1 Gyr (dotted lines), and therefore is not a major
mass ratios ≤ 1:10 are detected by these methods (same contributor to the uncertainty in Tobs (A, z) . However,
as Fig. 7). The weights for close pairs are normalized to the Tobs (A, z) calculated with the S08 weights (black
baryonic mass ratios between 1:1 and 1:4 or between 1:1 lines) are significantly higher than those calculated with
and 1:2, depending on the stellar mass and luminosity the C06 (blue lines) and St09 (red lines) weights. This
ratios adopted by the studies presented here (Table 1). arises from the strong dependence of A timescales on the
Both the gas fraction and morphological make-up of fgas and the different predictions for the distribution of
merger progenitors are expected to change significantly fgas (z) for the different models. The strong evolution
between 0 < z < 1.5. The number density of red of the mean fgas and higher frequency of very gas-rich
sequence (presumably gas-poor) galaxies more massive minor mergers predicted by S08 result in the strong evo-
than ∼ 1010 M⊙ roughly doubles over this time period lution of Tobs (A, z) with redshift. The low mean fgas
(Bell et al. 2004, Faber et al. 2007), and the fraction of and its weak evolution predicted by C06 results in short
bright galaxies which are spheroid dominated increases Tobs (A, z) that evolves weakly with redshift. The St09
from ∼ 20% at z ∼ 1.1 to ∼ 40% at z ∼ 0.3 (Lotz et models predict similar evolution in the mean fgas to S08,
al. 2008a). The cold gas fractions measured for small but has fewer high gas fraction intermediate/minor merg-
numbers of disk galaxies at z ∼ 1 − 1.5 are ∼ 40%, three ers, and therefore predicts lower values for Tobs (A, z) .
times higher than today’s ∼ 10% (Tacconi et al. 2010; In summary, the average observability timescales for
Daddi et al. 2010; McGaugh 2005). G − M20 and close pairs are not expected to evolve with
For G − M20 and close pairs, the individual observabil- redshift between 0 < z < 1.5, and are relatively insensi-
ity timescales Ti,j are not a strong function of fgas . We tive to the distribution of merger properties. The average
assume that fgas < 0.1 mergers have the same observabil- observability timescales for A, on the other hand, are ex-
ity timescales as fgas = 0.2 mergers of the same mass ra- pected to increase between 0 < z < 1.5 as the mean gas
tio. We note that gas-free spheroid-spheroid merger sim- fraction of mergers increase, and are highly sensitive to
ulations presented in Bell et al. (2005) imply ∼0.2 Gyr both the distribution of merger gas fractions and mass
timescales for visual classification of double nuclei, con- ratios. If the typical gas fractions of galaxy mergers at
sistent with our adopted G−M20 timescale for fgas < 0.1. z ≥ 1 are as high as 0.4, as suggested by recent molecular
However, the A timescales are a strong function of gas observations, then typical timescales for identifying a
fgas . Therefore, we compute upper and lower limits to merger as asymmetric should more than double. If very
Tobs (z) by assuming that that fgas < 0.1 mergers have gas-rich minor mergers are also more likely at z ≥ 1,
the same observability timescales as fgas = 0.2 merg- than the contribution of minor mergers to the asymmet-
ers (Figure 9; upper limit, solid lines) and by assuming ric galaxy population will also increase with redshift.
that fgas < 0.1 mergers are not detected by A (Fig-
ure 9: lower limit, dotted lines). We note that Bell
et al. (2006) found visual-classification timescales for 5. GALAXY MERGER RATES
gas-poor spheroid-spheroid major merger simulations of In the previous section, we calculated cosmologically-
Tobs ∼ 0.15 Gyr, intermediate between the Tobs = 0 and averaged merger observability timescales that account
Tobs ∼ 0.3 Gyr for fgas = 0 adopted here. for differences in merger identification techniques and
The cosmologically-averaged timescales for close pairs the distribution of merger mass ratio and gas fraction.
do not evolve with redshift and do not depend on the In this section, we use these improved estimates of
adopted cosmological galaxy evolution model (Fig. 8). Tobs (z) to re-analyze recent studies of the evolution
This is not surprising given that the close pair timescales of the galaxy mergers (reviewed in §3). In Tables 1, 2,
do not depend on gas fraction (which evolves strongly and 3, we give Tobs (z) calculated specifically for each
with redshift). The individual close pair timescales do merger study/technique, the resulting fractional merger
depend on mass ratio, but the relative distribution of rates, ℜmerg (z), and the merger rates per co-moving vol-