18 research outputs found
Gas Accretion as a Dominant Formation Mode in Massive Galaxies from the GOODS NICMOS Survey
The ability to resolve all processes which drive galaxy formation is one of
the most fundamental goals in extragalactic astronomy. While star formation
rates and the merger history are now measured with increasingly high certainty,
the role of gas accretion from the intergalactic medium in supplying gas for
star formation still remains largely unknown. We present in this paper indirect
evidence for the accretion of gas into massive galaxies with initial stellar
masses M_*>10^{11} M_sol and following the same merger adjusted co-moving
number density at lower redshifts during the epoch 1.5 < z < 3, using results
from the GOODS NICMOS Survey (GNS). We show that the measured gas mass
fractions of these massive galaxies are inconsistent with the observed star
formation history for the same galaxy population. We further demonstrate that
this additional gas mass cannot be accounted for by cold gas delivered through
minor and major mergers. We also consider the effects of gas outflows and gas
recycling due to stellar evolution in these calculations. We argue that to
sustain star formation at the observed rates there must be additional methods
for increasing the cold gas mass, and that the likeliest method for
establishing this supply of gas is by accretion from the intergalactic medium.
We calculate that the average gas mass accretion rate into these massive
galaxies between 1.5 < z < 3.0, is \dot{M} = 96+/-19 M_sol/yr after accounting
for outflowing gas. We show that during this epoch, and for these very massive
galaxies, 49+/-20% of baryonic mass assembly is a result of gas accretion and
unresolved mergers. However, 66+/-20% of all star formation in this epoch is
the result of gas accretion. This reveals that for the most massive galaxies at
1.5< z< 3 gas accretion is the dominant method for instigating new stellar mass
assembly.Comment: MNRAS in press, 11 pages, 5 figure
A Deep Probe of the Galaxy Stellar Mass Functions at z~1-3 with the GOODS NICMOS Survey
We use a sample of 8298 galaxies observed in the HST GOODS NICMOS Survey
(GNS) to construct the galaxy stellar mass function as a function of both
redshift and stellar mass up to z=3.5 and down to masses of Mstar=10^8.5 Msun
at z~1. We discover that a significant fraction of all massive Mstar>10^11 Msun
galaxies are in place up to the highest redshifts we probe, with a decreasing
fraction of lower mass galaxies present at all redshifts. This is an example of
`galaxy mass downsizing', and is the result of massive galaxies forming before
lower mass ones, and not just simply ending their star formation earlier as in
traditional downsizing scenarios. We find that the faint end slope is
significantly steeper than what is found in previous investigations. We
demonstrate that this steeper mass function better matches the stellar mass
added due to star formation, thereby alleviating some of the mismatch between
these two measures of the evolution of galaxy mass. We furthermore examine the
stellar mass function divided into blue/red systems, as well as for star
forming and non-star forming galaxies. We find a similar mass downsizing
present for both blue/red and star-forming/non-star forming galaxies, and that
the low mass galaxies are mostly all blue, and are therefore creating the steep
mass functions. We furthermore show that, although there is a downsizing such
that high mass galaxies are nearer their z=0 values at high redshift, this
turns over at masses Mstar~10^10 Msun, such that the lowest mass galaxies are
more common than galaxies at slight higher masses, creating a `dip' in the
observed galaxy mass function. We argue that the galaxy assembly process may be
driven by different mechanisms at low and high masses, and that the efficiency
of the galaxy formation process is lowest at masses Mstar~10^10 Msun at 1<z<3.
(Abridged)Comment: 16 pages, 11 figures, MNRAS, accepte
A WFC3 Grism Emission Line Redshift Catalog in the GOODS-South Field
We combine HST/WFC3 imaging and G141 grism observations from the CANDELS and
3D-HST surveys to produce a catalog of grism spectroscopic redshifts for
galaxies in the CANDELS/GOODS-South field. The WFC3/G141 grism spectra cover a
wavelength range of 1.1<lambda<1.7 microns with a resolving power of R~130 for
point sources, thus providing rest-frame optical spectra for galaxies out to
z~3.5. The catalog is selected in the H-band (F160W) and includes both galaxies
with and without previously published spectroscopic redshifts. Grism spectra
are extracted for all H-band detected galaxies with H<24 and a CANDELS
photometric redshift z_phot > 0.6. The resulting spectra are visually inspected
to identify emission lines and redshifts are determined using cross-correlation
with empirical spectral templates. To establish the accuracy of our redshifts,
we compare our results against high-quality spectroscopic redshifts from the
literature. Using a sample of 411 control galaxies, this analysis yields a
precision of sigma_NMAD=0.0028 for the grism-derived redshifts, which is
consistent with the accuracy reported by the 3D-HST team. Our final catalog
covers an area of 153 square arcmin and contains 1019 redshifts for galaxies in
GOODS-S. Roughly 60% (608/1019) of these redshifts are for galaxies with no
previously published spectroscopic redshift. These new redshifts span a range
of 0.677 < z < 3.456 and have a median redshift of z=1.282. The catalog
contains a total of 234 new redshifts for galaxies at z>1.5. In addition, we
present 20 galaxy pair candidates identified for the first time using the grism
redshifts in our catalog, including four new galaxy pairs at z~2, nearly
doubling the number of such pairs previously identified.Comment: 25 Pages, 9 Figures, submitted to A
The structures and total (minor + major) merger histories of massive galaxies up to z = 3 in the HST GOODS NICMOS Survey: A possible solution to the size evolution problem
We investigate the total major (> 1:4 by stellar mass) and minor (> 1:100 by
stellar mass) merger history of a population of 80 massive (M_* > 10^11 M_sol)
galaxies at high redshifts (z = 1.7 - 3). We utilize extremely deep and high
resolution HST H-band imaging from the GOODS NICMOS Survey (GNS), which
corresponds to rest-frame optical wavelengths at the redshifts probed. We find
that massive galaxies at high redshifts are often morphologically disturbed,
with a CAS deduced merger fraction f_m = 0.23 +/- 0.05 at z = 1.7 - 3. We find
close accord between close pair methods (within 30 kpc apertures) and CAS
methods for deducing major merger fractions at all redshifts. We deduce the
total (minor + major) merger history of massive galaxies with M_* > 10^9 M_sol
galaxies, and find that this scales roughly linearly with log-stellar-mass and
magnitude range. We test our close pair methods by utilizing mock galaxy
catalogs from the Millennium Simulation. We compute the total number of mergers
to be (4.5 +/- 2.9) / from z = 3 to the present, to a stellar mass
sensitivity threshold of ~ 1:100 (where \tau_m is the merger timescale in Gyr
which varies as a function of mass). This corresponds to an average mass
increase of (3.4 +/- 2.2) x 10^11 M_sol over the past 11.5 Gyrs due to merging.
We show that the size evolution observed for these galaxies may be mostly
explained by this merging.Comment: 19 pages, 10 figures, re-submitted to ApJ after a positive referee
report, originally submitted on Sept 20 201
Suppression of Star Formation in the central 200 kpc of a z = 1.4 Galaxy Cluster [Erratum added]
We present the results of an extended narrow-band H{\alpha} study of the
massive galaxy cluster XMMU J2235.3-2557 at z = 1.39. This paper represents a
follow up study to our previous investigation of star-formation in the cluster
centre, extending our analysis out to a projected cluster radius of 1.5 Mpc.
Using the Near InfraRed Imager and Spectrograph (NIRI) on Gemini North we
obtained deep H narrow-band imaging corresponding to the rest-frame wavelength
of H{\alpha} at the cluster's redshift. We identify a total of 163 potential
cluster members in both pointings, excluding stars based on their near-IR
colours derived from VLT/HAWK-I imaging. Of these 163 objects 14 are
spectroscopically confirmed cluster members, and 20% are excess line-emitters.
We find no evidence of star formation activity within a radius of 200 kpc of
the brightest cluster galaxy in the cluster core. Dust-corrected star formation
rates (SFR) of excess emitters outside this cluster quenching radius, RQ \sim
200 kpc, are on average = 2.7 \pm 1.0 M\odot yr-1, but do not show
evidence of increasing star-formation rates toward the extreme 1.5 Mpc radius
of the cluster. No individual cluster galaxy exceeds an SFR of 6 M\odot yr-1 .
Massive galaxies (log M\ast /M\odot > 10.75) all have low specific SFRs (SSFRs,
i.e. SFR per unit stellar mass). At fixed stellar mass, galaxies in the cluster
centre have lower SSFRs than the rest of the cluster galaxies, which in turn
have lower SSFRs than field galaxies at the same redshift by a factor of a few
to 10. For the first time we can demonstrate through measurements of individual
SFRs that already at very early epochs (at an age of the Universe of \sim4.5
Gyr) the suppression of star-formation is an effect of the cluster environment
which persists at fixed galaxy stellar mass. [Erratum added after the original
paper]Comment: 12 pages, 6 figures, accepted for publication in MNRAS. New version:
Erratum added after the original paper (2 pages, 3 corrected figures). Due to
an error in the original computation the SFR values had to be increased by a
factor of 5. However, the results and conclusions remain largely unchange
Galaxy Zoo: CANDELS barred discs and bar fractions
The formation of bars in disc galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in discs decreases from the local Universe to z ~ 1, and by z > 1 simulations predict that bar features in dynamically mature discs should be extremely rare. Here, we report the discovery of strong barred structures in massive disc galaxies at z ~ 1.5 in deep rest-frame optical images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. From within a sample of 876 disc galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a subsample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5 ≤ z ≤ 2 (fbar = 10.7+6.3 -3.5 per cent after correcting for incompleteness) does not significantly evolve.We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disc galaxies have evolved over the last 11 billion yearsPeer reviewedFinal Accepted Versio
Measures of Galaxy Environment - I. What is "Environment"?
The influence of a galaxy's environment on its evolution has been studied and
compared extensively in the literature, although differing techniques are often
used to define environment. Most methods fall into two broad groups: those that
use nearest neighbours to probe the underlying density field and those that use
fixed apertures. The differences between the two inhibit a clean comparison
between analyses and leave open the possibility that, even with the same data,
different properties are actually being measured. In this work we apply twenty
published environment definitions to a common mock galaxy catalogue constrained
to look like the local Universe. We find that nearest neighbour-based measures
best probe the internal densities of high-mass haloes, while at low masses the
inter-halo separation dominates and acts to smooth out local density
variations. The resulting correlation also shows that nearest neighbour galaxy
environment is largely independent of dark matter halo mass. Conversely,
aperture-based methods that probe super-halo scales accurately identify
high-density regions corresponding to high mass haloes. Both methods show how
galaxies in dense environments tend to be redder, with the exception of the
largest apertures, but these are the strongest at recovering the background
dark matter environment. We also warn against using photometric redshifts to
define environment in all but the densest regions. When considering environment
there are two regimes: the 'local environment' internal to a halo best measured
with nearest neighbour and 'large-scale environment' external to a halo best
measured with apertures. This leads to the conclusion that there is no
universal environment measure and the most suitable method depends on the scale
being probed.Comment: 14 pages, 9 figures, 1 table, published in MNRA
Galaxy Zoo: CANDELS barred discs and bar fractions
The formation of bars in disc galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in discs decreases from the local Universe to z ∼ 1, and by z>1 simulations predict that bar features in dynamically mature discs should be extremely rare. Here, we report the discovery of strong barred structures in massive disc galaxies at z ∼ 1.5 in deep rest-frame optical images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. From within a sample of 876 disc galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a subsample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5 ≤ z ≤ 2 ( per cent after correcting for incompleteness) does not significantly evolve. We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disc galaxies have evolved over the last 11 billion year