158 research outputs found
The Evolution of the Number Density of Large Disk Galaxies in COSMOS
We study a sample of approximately 16,500 galaxies with I_(ACS,AB) †22.5 in the central 38% of the COSMOS field, which are extracted from a catalog constructed from the Cycle 12 ACS F814W COSMOS data set. Structural information on the galaxies is derived by fitting single SĂ©rsic models to their two-dimensional surface brightness distributions. In this paper we focus on the disk galaxy population (as classified by the Zurich Estimator of Structural Types), and investigate the evolution of the number density of disk galaxies larger than approximately 5 kpc between redshift z ~ 1 and the present epoch. Specifically, we use the measurements of the half-light radii derived from the SĂ©rsic fits to construct, as a function of redshift, the size function Ί(r_(1/2), z) of both the total disk galaxy population and of disk galaxies split in four bins of bulge-to-disk ratio. In each redshift bin, the size function specifies the number of galaxies per unit comoving volume and per unit half-light radius r_(1/2). Furthermore, we use a selected sample of roughly 1800 SDSS galaxies to calibrate our results with respect to the local universe. We find the following: (1) The number density of disk galaxies with intermediate sizes (r_(1/2) ~ 5-7 kpc) remains nearly constant from z ~ 1 to today. Unless the growth and destruction of such systems exactly balanced in the last eight billion years, they must have neither grown nor been destroyed over this period. (2) The number density of the largest disks (r_(1/2) > 7 kpc) decreases by a factor of about 2 out to z ~ 1. (3) There is a constancyâor even slight increaseâin the number density of large bulgeless disks out to z ~ 1; the deficit of large disks at early epochs seems to arise from a smaller number of bulged disks. Our results indicate that the bulk of the large disk galaxy population has completed its growth by z ~ 1 and support the theory that secular evolution processes produceâor at least add stellar mass toâthe bulge components of disk galaxies
COSMOS morphological classification with ZEST (the Zurich Estimator of Structural Types) and the evolution since z=1 of the Luminosity Function of early-, disk-, and irregular galaxies
(ABRIDGED) Motivated by the desire to reliably and automatically classify
structure of thousands of COSMOS galaxies, we present ZEST, the Zurich
Estimator of Structural Types. To classify galaxy structure, ZEST uses: (i)
Five non-parametric diagnostics: asymmetry, concentration, Gini coefficient,
2nd-order moment of the brightest 20% of galaxy pixels, and ellipticity; and
(ii) The exponent n of single--Sersic fits to the 2D surface brightness
distributions. To fully exploit the wealth of information while reducing the
redundancy present in these diagnostics, ZEST performs a principal component
(PC) Analysis. We use a sample of ~56,000 I<24 COSMOS galaxies to show that the
first three PCs fully describe the key aspects of the galaxy structure, i.e.,
to calibrate a three-dimensional classification grid of axis PC_1, PC_2, and
PC_3. We demonstrate the robustness of the ZEST grid on the z=0 sample of Frei
et al. (1996). The ZEST classification breaks most of the degeneracy between
different galaxy populations that affects morphological classifications based
on only some of the diagnostics included in ZEST. As a first application, we
present the evolution since z~1 of the Luminosity Functions of COSMOS galaxies
of early, disk and irregular galaxies and, for disk galaxies, of different
bulge-to-disk ratios. Overall, we find that the LF up to a redshift z=1 is
consistent with a pure-luminosity evolution (of about 0.95 magnitudes at z
\~0.7). We highlight however two trends, that are in general agreement with a
down-sizing scenario for galaxy formation: (1.) A deficit of a factor of about
two at z~0.7 of MB>-20.5 structurally--classified early--type galaxies; and
(2.) An excess of a factor of about three, at a similar redshift, of irregular
galaxies.Comment: Accepted for publication in the ApJ COSMOS special issue. A version
with high resolution figures is available at
http://www.exp-astro.phys.ethz.ch/scarlata/papers/ApJS_ZEST.pd
A group-galaxy cross-correlation function analysis in zCOSMOS
We present a group-galaxy cross-correlation analysis using a group catalog
produced from the 16,500 spectra from the optical zCOSMOS galaxy survey. Our
aim is to perform a consistency test in the redshift range 0.2 < z < 0.8
between the clustering strength of the groups and mass estimates that are based
on the richness of the groups. We measure the linear bias of the groups by
means of a group-galaxy cross-correlation analysis and convert it into mass
using the bias-mass relation for a given cosmology, checking the systematic
errors using realistic group and galaxy mock catalogs. The measured bias for
the zCOSMOS groups increases with group richness as expected by the theory of
cosmic structure formation and yields masses that are reasonably consistent
with the masses estimated from the richness directly, considering the scatter
that is obtained from the 24 mock catalogs. An exception are the richest groups
at high redshift (estimated to be more massive than 10^13.5 M_sun), for which
the measured bias is significantly larger than for any of the 24 mock catalogs
(corresponding to a 3-sigma effect), which is attributed to the extremely large
structure that is present in the COSMOS field at z ~ 0.7. Our results are in
general agreement with previous studies that reported unusually strong
clustering in the COSMOS field.Comment: 13 pages, 9 figures, published in Ap
zCOSMOS 20k: Satellite galaxies are the main drivers of environmental effects in the galaxy population at least to z~0.7
We explore the role of environment in the evolution of galaxies over
0.1<z<0.7 using the final zCOSMOS-bright data set. Using the red fraction of
galaxies as a proxy for the quenched population, we find that the fraction of
red galaxies increases with the environmental overdensity and with the stellar
mass, consistent with previous works. As at lower redshift, the red fraction
appears to be separable in mass and environment, suggesting the action of two
processes: mass and environmental quenching. The parameters describing these
appear to be essentially the same at z~0.7 as locally. We explore the relation
between red fraction, mass and environment also for the central and satellite
galaxies separately, paying close attention to the effects of impurities in the
central-satellite classification and using carefully constructed samples
matched in stellar mass. There is little evidence for a dependence of the red
fraction of centrals on overdensity. Satellites are consistently redder at all
overdensities, and the satellite quenching efficiency increases with
overdensity at 0.1<z<0.4. This is less marked at higher redshift, but both are
nevertheless consistent with the equivalent local measurements. At a given
stellar mass, the fraction of galaxies that are satellites also increases with
the overdensity. At a given overdensity and mass, the obtained relation between
the environmental quenching and the satellite fraction agrees well with the
satellite quenching efficiency, demonstrating that the environmental quenching
in the overall population is consistent with being entirely produced through
the satellite quenching process at least up to z=0.7. However, despite the
unprecedented size of our high redshift samples, the associated statistical
uncertainties are still significant and our statements should be understood as
approximations to physical reality, rather than physically exact formulae.Comment: 22 pages, 19 figures, submitted to MNRA
The dependence of Galactic outflows on the properties and orientation of zCOSMOS galaxies at z ~ 1
We present an analysis of cool outflowing gas around galaxies, traced by MgII
absorption lines in the co-added spectra of a sample of 486 zCOSMOS galaxies at
1 < z < 1.5. These galaxies span a range of stellar masses (9.45<
log[M*/Msun]<10.7) and star formation rates (0.14 < log [SFR/Msun/yr] < 2.35).
We identify the cool outflowing component in the MgII absorption and find that
the equivalent width of the outflowing component increases with stellar mass.
The outflow equivalent width also increases steadily with the increasing star
formation rate of the galaxies. At similar stellar masses the blue galaxies
exhibit a significantly higher outflow equivalent width as compared to red
galaxies. The outflow equivalent width shows strong effect with star formation
surface density ({\Sigma}SFR) of the sample. For the disk galaxies, the outflow
equivalent width is higher for the face-on systems as compared to the edge-on
ones, indicating that for the disk galaxies, the outflowing gas is primarily
bipolar in geometry. Galaxies typically exhibit outflow velocities ranging from
-200 km/s to -300 km/s and on average the face-on galaxies exhibit higher
outflow velocity as compared to the edge-on ones. Galaxies with irregular
morphologies exhibit outflow equivalent width as well as outflow velocities
comparable to face on disk galaxies. These galaxies exhibit minimum mass
outflow rates > 5-7 Msun/yr and a mass loading factor ({\eta} = dMout/dt /SFR)
comparable to the star formation rates of the galaxies.Comment: 12 pages, 14 figures, ApJ submitte
K+a galaxies in the zCOSMOS Survey: Physical properties of systems in their post-starburst phase
The identities of the main processes triggering and quenching star-formation
in galaxies remain unclear. A key stage in evolution, however, appears to be
represented by post-starburst galaxies. To investigate their impact on galaxy
evolution, we initiated a multiwavelength study of galaxies with k+a spectral
features in the COSMOS field. We examine a mass-selected sample of k+a galaxies
at z=0.48-1.2 using the spectroscopic zCOSMOS sample. K+a galaxies occupy the
brightest tail of the luminosity distribution. They are as massive as quiescent
galaxies and populate the green valley in the colour versus luminosity (or
stellar mass) distribution. A small percentage (<8%) of these galaxies have
radio and/or X-ray counterparts (implying an upper limit to the SFR of
~8Msun/yr). Over the entire redshift range explored, the class of k+a galaxies
is morphologically a heterogeneous population with a similar incidence of
bulge-dominated and disky galaxies. This distribution does not vary with the
strength of the Hdelta absorption line but instead with stellar mass in a way
reminiscent of the well-known mass-morphology relation. Although k+a galaxies
are also found in underdense regions, they appear to reside typically in a
similarly rich environment as quiescent galaxies on a physical scale of
~2-8Mpc, and in groups they show a morphological early-to-late type ratio
similar to the quiescent galaxy class. With the current data set, we do not
find evidence of statistical significant evolution in either the number/mass
density of k+a galaxies at intermediate redshift with respect to the local
values, or the spectral properties. Those galaxies, which are affected by a
sudden quenching of their star-formation activity, may increase the stellar
mass of the red-sequence by up to a non-negligible level of ~10%.Comment: 17 pages, 9 figures. Accepted for publication in Astronomy and
Astrophysics on 09/09/2009 (no changes wrt v1
Extreme emission-line galaxies out to z1 in zCOSMOS. I. Sample and characterization of global properties
We present a thorough characterization of a large sample of 183 extreme
emission-line galaxies (EELGs) at redshift 0.11 < z < 0.93 selected from the
20k zCOSMOS Bright Survey because of their unusually large emission line
equivalent widths. We use multiwavelength COSMOS photometry, HST-ACS I-band
imaging and optical zCOSMOS spectroscopy to derive the main global properties
of EELGs, such as sizes, masses, SFRs, reliable metallicities from both
"direct" and "strong-line" methods. The EELGs are compact (R_50 ~ 1.3 kpc),
low-mass (log(M*/Msol)~7-10) galaxies forming stars at unusually high specific
SFR (log(sSFR/yr) up to ~ -7) compared to main sequence SFGs of the same
stellar mass and redshift. At UV wavelengths, the EELGs are luminous and show
high surface brightness and include strong Ly emitters, as revealed by
GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization,
low-metallicity systems, with median 12+log(O/H)=8.16, including a handful of
extremely metal-deficient galaxies (<10% solar). While ~80% of the EELGs show
non-axisymmetric morphologies, including clumpy and tadpole galaxies, we find
that ~29% of them show additional low surface-brightness features, which
strongly suggest recent or ongoing interactions. As star-forming dwarfs in the
local Universe, EELGs are most often found in relative isolation. While only
very few EELGs belong to compact groups, almost one third of them are found in
spectroscopically confirmed loose pairs or triplets. We conclude that EELGs are
galaxies caught in a transient and probably early period of their evolution,
where they are efficiently building-up a significant fraction of their
present-day stellar mass in an ongoing galaxy-wide starburst. Therefore, the
EELGs constitute an ideal benchmark for comparison studies between low- and
high-redshift low-mass star-forming galaxies.Comment: Accepted in A&A. Final replacement to match the version in press. It
includes a minor change in the title and a new figur
The zCOSMOS 20k Group Catalog
We present an optical group catalog between 0.1 < z < 1 based on 16,500
high-quality spectroscopic redshifts in the completed zCOSMOS-bright survey.
The catalog published herein contains 1498 groups in total and 192 groups with
more than five observed members. The catalog includes both group properties and
the identification of the member galaxies. Based on mock catalogs, the
completeness and purity of groups with three and more members should be both
about 83% with respect to all groups that should have been detectable within
the survey, and more than 75% of the groups should exhibit a one-to-one
correspondence to the "real" groups. Particularly at high redshift, there are
apparently more galaxies in groups in the COSMOS field than expected from mock
catalogs. We detect clear evidence for the growth of cosmic structure over the
last seven billion years in the sense that the fraction of galaxies that are
found in groups (in volume-limited samples) increases significantly with cosmic
time. In the second part of the paper, we develop a method for associating
galaxies that only have photo-z to our spectroscopically identified groups. We
show that this leads to improved definition of group centers, improved
identification of the most massive galaxies in the groups, and improved
identification of central and satellite galaxies, where we define the former to
be galaxies at the minimum of the gravitational potential wells. Subsamples of
centrals and satellites in the groups can be defined with purities up to 80%,
while a straight binary classification of all group and non-group galaxies into
centrals and satellites achieves purities of 85% and 75%, respectively, for the
spectroscopic sample.Comment: 26 pages, 21 figures, published in ApJ (along with machine-readable
tables
Simulating the COSMOS: The fraction of merging galaxies at high redshift
Simulations of nearby (0.015 < z < 0.025) SDSS galaxies have been used to
reproduce as accurately as possible the appearance that they would have on
COSMOS ACS images if they had been observed at z ~ 0.7 and z ~ 1.2. By adding
the SDSS galaxies to random locations in the COSMOS images, we simulate the
effects of chance superpositions of high redshift galaxies with unrelated
foreground or background objects. We have used these simulated images, together
with those of real COSMOS galaxies at these same redshifts, to undertake a
"blind" morphological classification of galaxies to identify those that appear
to be undergoing mergers and thus to estimate the change in merger fraction
with redshift. We find that real mergers are harder to recognize at high
redshift, and also that the chance superposition of unrelated galaxies often
produces the appearance of mergers where in reality none exists. In particular,
we estimate that 1.5 - 2.0% of objects randomly added to ACS images are
misclassified as mergers due to projection with unrelated objects, and as a
result, that 40% of the apparent mergers in COSMOS at z=0.7 are likely to be
spurious. We find that the fraction of galaxies undergoing mergers increases as
(1+z)^3.8+/-1.2 to z ~ 0.7 and that this trend appears to continue to z = 1.2.
Merger candidates at z ~ 0.7 are bluer than the parent population, especially
when the statistical effects of the chance projections are accounted for.
Merger candidates are more asymmetric than the population as a whole, and are
often associated with irregular morphology. Nevertheless, the majority (~60%)
of the merger candidates appear to be associated with spiral galaxies although
in this case we cannot correct for the effects of chance projections.Comment: 24 pages, 7 figures; accepted for publication in the Ap
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