49 research outputs found
Absence of Evidence Is Not Evidence of Absence: The Color-Density Relation at Fixed Stellar Mass Persists to z ~ 1
We use data drawn from the DEEP2 Galaxy Redshift Survey to investigate the
relationship between local galaxy density, stellar mass, and rest-frame galaxy
color. At z ~ 0.9, we find that the shape of the stellar mass function at the
high-mass (log (M*/Msun) > 10.1) end depends on the local environment, with
high-density regions favoring more massive systems. Accounting for this stellar
mass-environment relation (i.e., working at fixed stellar mass), we find a
significant color-density relation for galaxies with 10.6 < log(M*/Msun) < 11.1
and 0.75 < z < 0.95. This result is shown to be robust to variations in the
sample selection and to extend to even lower masses (down to log(M*/Msun) ~
10.4). We conclude by discussing our results in comparison to recent works in
the literature, which report no significant correlation between galaxy
properties and environment at fixed stellar mass for the same redshift and
stellar mass domain. The non-detection of environmental dependence found in
other data sets is largely attributable to their smaller samples size and lower
sampling density, as well as systematic effects such as inaccurate redshifts
and biased analysis techniques. Ultimately, our results based on DEEP2 data
illustrate that the evolutionary state of a galaxy at z ~ 1 is not exclusively
determined by the stellar mass of the galaxy. Instead, we show that local
environment appears to play a distinct role in the transformation of galaxy
properties at z > 1.Comment: 10 pages, 5 Figures; Accepted for publication in MNRA
The Rise and Fall of Passive Disk Galaxies: Morphological Evolution Along the Red Sequence Revealed by COSMOS
The increasing abundance of passive "red-sequence" galaxies since z=1-2 is
mirrored by a coincident rise in the number of galaxies with spheroidal
morphologies. In this paper, however, we show that in detail the correspondence
between galaxy morphology and color is not perfect, providing insight into the
physical origin of this evolution. Using the COSMOS survey, we study a
significant population of red sequence galaxies with disk-like morphologies.
These passive disks typically have Sa-Sb morphological types with large bulges,
but they are not confined to dense environments. They represent nearly one-half
of all red-sequence galaxies and dominate at lower masses (log Mstar < 10)
where they are increasingly disk-dominated. As a function of time, the
abundance of passive disks with log Mstar < 11 increases, but not as fast as
red-sequence spheroidals in the same mass range. At higher mass, the passive
disk population has declined since z~1, likely because they transform into
spheroidals. We estimate that as much as 60% of galaxies transitioning onto the
red sequence evolve through a passive disk phase. The origin of passive disks
therefore has broad implications for understanding how star formation shuts
down. Because passive disks tend to be more bulge-dominated than their
star-forming counterparts, a simple fading of blue disks does not fully explain
their origin. We explore several more sophisticated explanations, including
environmental effects, internal stabilization, and disk regrowth during
gas-rich mergers. While previous work has sought to explain color and
morphological transformations with a single process, these observations open
the way to new insight by highlighting the fact that galaxy evolution may
actually proceed through several separate stages.Comment: 16 pages, Accepted version to appear in Ap
Backsplash galaxies in isolated clusters
At modest radii from the centre of galaxy clusters, individual galaxies may
be infalling to the cluster for the first time, or have already visited the
cluster core and are coming back out again. This latter population of galaxies
is known as the backsplash population. Differentiating them from the infalling
population presents an interesting challenge for observational studies of
galaxy evolution. To attempt to do this, we assemble a sample of 14 redshift-
and spatially-isolated galaxy clusters from the Sloan Digital Sky Survey. We
clean this sample of cluster-cluster mergers to ensure that the galaxies
contained within them are (to an approximation) only backsplashing from the
centre of their parent clusters and are not being processed in sub-clumps. By
stacking them together to form a composite cluster, we find evidence for both
categories of galaxies at intermediate radii from the cluster centre.
Application of mixture modelling to this sample then serves to differentiate
the infalling galaxies (which we model on galaxies from the cluster outskirts)
from the backsplash ones (which we model on galaxies in the high density core
with low velocity offsets from the cluster mean). We find that the fraction of
galaxies with populations similar to the low velocity cluster core galaxies is
f = -0.052R/R_virial + 0.612 +/- 0.06 which we interpret as being the
backsplash population fraction at 1<R/R_virial<2. Although some interlopers may
be affecting our results, the results are demonstrated to be in concordance
with earlier studies in this area that support density-related mechanisms as
being the prime factor in determining the star formation rate of a galaxy.Comment: Accepted for publication in MNRA
Compact High-Redshift Galaxies Are the Cores of the Most Massive Present-Day Spheroids
Observations suggest that effective radii of high-z massive spheroids are as
much as a factor ~6 smaller than low-z galaxies of comparable mass. Given the
apparent absence of low-z counterparts, this has often been interpreted as
indicating that the high density, compact red galaxies must be 'puffed up' by
some mechanism. We compare the ensemble of high-z observations with large
samples of well-observed low-z ellipticals. At the same physical radii, the
stellar surface mass densities of low and high-z systems are comparable.
Moreover, the abundance of high surface density material at low redshift is
comparable to or larger than that observed at z>1-2, consistent with the
continuous buildup of spheroids over this time. The entire population of
compact, high-z red galaxies may be the progenitors of the high-density cores
of present-day ellipticals, with no need for a decrease in stellar density from
z=2 to z=0. The primary difference between low and high-z systems is thus the
observed low-density material at large radii in low-z spheroids (rather than
the high-density material in high-z spheroids). Such low-density material may
either (1) assemble at z2.
Mock observations of low-z massive systems show that the high-z observations do
not yet probe sufficiently low surface brightness material to detect the low
surface density 'wings' (if present). Thus, if the high-z galaxies resemble the
most massive systems today, their inferred effective radii could be
under-estimated by factors ~2-4. This difference arises because massive systems
at low redshift are not well-fit by single Sersic profiles. We discuss
implications of our results for physical models of galaxy evolution.Comment: 14 pages, 6 figures, accepted to MNRAS (revised to match published
version
Resolving the far-IR line deficit : photoelectric heating and far-IR line cooling in NGC 1097 and NGC 4559
The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 mu m, [O I] 63 mu m, and [N II] 122 mu m in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, nu f(nu)(70 mu m) nu f(nu)(100 mu m), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant similar to 6% over a wide range of far-IR color, 0.5 , derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 mu m PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments
The emission by dust and stars of nearby galaxies in the Herschel KINGFISH survey
Using new far-infrared imaging from the Herschel Space Observatory with ancillary data from ultraviolet (UV) to submillimeter wavelengths, we estimate the total emission from dust and stars of 62 nearby galaxies in the KINGFISH survey in a way that is as empirical and model independent as possible. We collect and exploit these data in order to measure from the spectral energy distributions (SEDs) precisely how much stellar radiation is intercepted and re-radiated by dust, and how this quantity varies with galaxy properties. By including SPIRE data, we are more sensitive to emission from cold dust grains than previous analyses at shorter wavelengths, allowing for more accurate estimates of dust temperatures and masses. The dust/stellar flux ratio, which we measure by integrating the SEDs, has a range of nearly three decades (from 10(-2.2) to 10(0.5)). The inclusion of SPIRE data shows that estimates based on data not reaching these far-IR wavelengths are biased low by 17% on average. We find that the dust/stellar flux ratio varies with morphology and total infrared (IR) luminosity, with dwarf galaxies having faint luminosities, spirals having relatively high dust/stellar ratios and IR luminosities, and some early types having low dust/stellar ratios. We also find that dust/stellar flux ratios are related to gas-phase metallicity ((log(f(dust)/f(*)) over bar) = -0.66 +/- 0.08 and -0.22 +/- 0.12 for metal-poor and intermediate-metallicity galaxies, respectively), while the dust/stellar mass ratios are less so (differing by approximate to 0.2 dex); the more metal-rich galaxies span a much wider range of the flux ratios. In addition, the substantial scatter between dust/stellar flux and dust/stellar mass indicates that the former is a poor proxy of the latter. Comparing the dust/stellar flux ratios and dust temperatures, we also show that early types tend to have slightly warmer temperatures (by up to 5 K) than spiral galaxies, which may be due to more intense interstellar radiation fields, or possibly to different dust grain compositions. Finally, we show that early types and early-type spirals have a strong correlation between the dust/stellar flux ratio and specific star formation rate, which suggests that the relatively bright far-IR emission of some of these galaxies is due to ongoing (if limited) star formation as well as to the radiation field from older stars, which is heating the dust grains
The Multiwavelength Survey by Yale-Chile (MUSYC): Deep Medium-Band optical imaging and high quality 32-band photometric redshifts in the ECDF-S
We present deep optical 18-medium-band photometry from the Subaru telescope
over the ~30' x 30' Extended Chandra Deep Field-South (ECDF-S), as part of the
Multiwavelength Survey by Yale-Chile (MUSYC). This field has a wealth of
ground- and space-based ancillary data, and contains the GOODS-South field and
the Hubble Ultra Deep Field. We combine the Subaru imaging with existing
UBVRIzJHK and Spitzer IRAC images to create a uniform catalog. Detecting
sources in the MUSYC BVR image we find ~40,000 galaxies with R_AB<25.3, the
median 5 sigma limit of the 18 medium bands. Photometric redshifts are
determined using the EAZY code and compared to ~2000 spectroscopic redshifts in
this field. The medium band filters provide very accurate redshifts for the
(bright) subset of galaxies with spectroscopic redshifts, particularly at 0.1 <
z 3.5. For 0.1 < z < 1.2, we find a 1 sigma scatter in \Delta
z/(1+z) of 0.007, similar to results obtained with a similar filter set in the
COSMOS field. As a demonstration of the data quality, we show that the red
sequence and blue cloud can be cleanly identified in rest-frame color-magnitude
diagrams at 0.1 < z < 1.2. We find that ~20% of the red-sequence-galaxies show
evidence of dust-emission at longer rest-frame wavelengths. The reduced images,
photometric catalog, and photometric redshifts are provided through the public
MUSYC website.Comment: 19 pages, 14 image
The Morphology of Galaxies in the Baryon Oscillation Spectroscopic Survey
We study the morphology of luminous and massive galaxies at 0.3<z<0.7
targeted in the Baryon Oscillation Spectroscopic Survey (BOSS) using publicly
available Hubble Space Telescope imaging from COSMOS. Our sample (240 objects)
provides a unique opportunity to check the visual morphology of these galaxies
which were targeted based solely on stellar population modelling. We find that
the majority (74+/-6%) possess an early-type morphology (elliptical or S0),
while the remainder have a late-type morphology. This is as expected from the
goals of the BOSS target selection which aimed to predominantly select slowly
evolving galaxies, for use as cosmological probes, while still obtaining a fair
fraction of actively star forming galaxies for galaxy evolution studies. We
show that a colour cut of (g-i)>2.35 selects a sub-sample of BOSS galaxies with
90% early-type morphology - more comparable to the earlier Luminous Red Galaxy
(LRG) samples of SDSS-I/II. The remaining 10% of galaxies above this cut have a
late-type morphology and may be analogous to the "passive spirals" found at
lower redshift. We find that 23+/-4% of the early-type galaxies are unresolved
multiple systems in the SDSS imaging. We estimate that at least 50% of these
are real associations (not projection effects) and may represent a significant
"dry merger" fraction. We study the SDSS pipeline sizes of BOSS galaxies which
we find to be systematically larger (by 40%) than those measured from HST
images, and provide a statistical correction for the difference. These details
of the BOSS galaxies will help users of the data fine-tune their selection
criteria, dependent on their science applications. For example, the main goal
of BOSS is to measure the cosmic distance scale and expansion rate of the
Universe to percent-level precision - a point where systematic effects due to
the details of target selection may become important.Comment: 18 pages, 11 figures; v2 as accepted by MNRA
Can Minor Merging Account for the Size Growth of Quiescent Galaxies? New Results from the CANDELS Survey
The presence of extremely compact galaxies at z~2 and their subsequent growth
in physical size has been the cause of much puzzlement. We revisit the question
using deep infrared Wide Field Camera 3 data to probe the rest-frame optical
structure of 935 host galaxies selected with 0.4
10^10.7 Msol using optical and near-infrared photometry in the UKIRT Ultra Deep
Survey and GOODS-South fields of the CANDELS survey. At each redshift, the most
compact sources are those with little or no star formation, and we find that
the mean size of these systems grows by a factor of 3.5 +- 0.3 over this
redshift interval. The new data are sufficiently deep to enable us to identify
companions to these hosts whose stellar masses are ten times smaller, while
still yielding suitably accurate photometric redshifts to define a likely
physical association. By searching for faint companions around 404 quiescent
hosts within a projected physical annulus 10 < R < 30 kpc/h, we estimate the
minor merger rate over the redshift range 0.4 < z < 2. After correcting for
contamination from projected pairs, we find that 13-18% of quiescent hosts have
likely physical companions with stellar mass ratios of 0.1 or greater. Mergers
of these companions will typically increase the host mass by 6+-2% per merger
timescale. We estimate the minimum growth rate necessary to explain the
declining abundance of compact galaxies. Using a simple model of merging
motivated by recent numerical simulations, we then assess whether mergers of
the faint companions with their hosts are sufficient to explain this minimal
rate. We find that mergers with mass ratios > 0.1 may explain most of the size
evolution observed at z >~ 1 if a relatively short merger timescale is assumed,
but the rapid growth seen at higher redshift likely requires additional
physical processes.Comment: Accepted to ApJ. Updated following referee report, with expanded
comparisons to published mass-size and pair fraction measurements (Figs. 4
and 9
Discriminating Between the Physical Processes that Drive Spheroid Size Evolution
Massive galaxies at high-z have smaller effective radii than those today, but
similar central densities. Their size growth therefore relates primarily to the
evolving abundance of low-density material. Various models have been proposed
to explain this evolution, which have different implications for galaxy, star,
and BH formation. We compile observations of spheroid properties as a function
of redshift and use them to test proposed models. Evolution in progenitor
gas-richness with redshift gives rise to initial formation of smaller spheroids
at high-z. These systems can then evolve in apparent or physical size via
several channels: (1) equal-density 'dry' mergers, (2) later major or minor
'dry' mergers with less-dense galaxies, (3) adiabatic expansion, (4) evolution
in stellar populations & mass-to-light-ratio gradients, (5) age-dependent bias
in stellar mass estimators, (6) observational fitting/selection effects. If any
one of these is tuned to explain observed size evolution, they make distinct
predictions for evolution in other galaxy properties. Only model (2) is
consistent with observations as a dominant effect. It is the only model which
allows for an increase in M_BH/M_bulge with redshift. Still, the amount of
merging needed is larger than that observed or predicted. We therefore compare
cosmologically motivated simulations, in which all these effects occur, & show
they are consistent with all the observational constraints. Effect (2), which
builds up an extended low-density envelope, does dominate the evolution, but
effects 1,3,4, & 6 each contribute ~20% to the size evolution (a net factor
~2). This naturally also predicts evolution in M_BH-sigma similar to that
observed.Comment: 19 pages, 7 figures. accepted to MNRAS (matches accepted version