158 research outputs found
Clustering of Very Red Galaxies in the Las Campanas IR Survey
We report results from the first 1000 square arc-minutes of the Las Campanas
IR survey. We have imaged 1 square degree of high latitude sky in six distinct
fields to a 5-sigma H-band depth of 20.5 (Vega). Optical imaging in the
V,R,I,and z' bands allow us to select color subsets and
photometric-redshift-defined shells. We show that the angular clustering of
faint red galaxies (18 3) is an order of magnitude stronger
than that of the complete H-selected field sample. We employ three approaches
to estimate in order to invert w(theta) to derive r_0. We find that our
n(z) is well described by a Gaussian with = 1.2, sigma(z) = 0.15. From this
we derive a value for r_0 of 7 (+2,-1) co-moving H^{-1} Mpc at = 1.2. This
is a factor of ~ 2 larger than the clustering length for Lyman break galaxies
and is similar to the expectation for early type galaxies at this epoch.Comment: 5 pages, 2 figures, 1 table. To appear in proceedings of the
ESO/ECF/STScI workshop "Deep Fields" held in Garching, Germany, 9-12 October
200
Evolved Galaxies at z > 1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems
We present spectroscopic evidence from the Gemini Deep Deep Survey (GDDS) for
a significant population of color-selected red galaxies at 1.3 < z < 2.2 whose
integrated light is dominated by evolved stars. Unlike radio-selected objects,
the z > 1.5 old galaxies have a sky density > 0.1 per sq. arcmin. Conservative
age estimates for 20 galaxies with z > 1.3; = 1.49, give a median age of
1.2 Gyr and = 2.4. One quarter of the galaxies have inferred z_f > 4.
Models restricted to abundances less than or equal to solar give median ages
and z_f of 2.3 Gyr and 3.3, respectively. These galaxies are among the most
massive and contribute approximately 50% of the stellar mass density at 1 < z <
2. The derived ages and most probable star formation histories suggest a high
star-formation-rate (300-500 solar masses per year) phase in the progenitor
population. We argue that most of the red galaxies are not descendants of the
typical z=3 Lyman break galaxies. Galaxies associated with luminous sub-mm
sources have the requisite star formation rates to be the progenitor
population. Our results point toward early and rapid formation for a
significant fraction of present day massive galaxies.Comment: 12 pages, 2 figures, 1 table, Accepted for publication, ApJ Letter
Cosmic Star Formation History and its Dependence on Galaxy Stellar Mass
We examine the cosmic star formation rate (SFR) and its dependence on galaxy
stellar mass over the redshift range 0.8 < z < 2 using data from the Gemini
Deep Deep Survey (GDDS). The SFR in the most massive galaxies (M > 10^{10.8}
M_sun) was six times higher at z = 2 than it is today. It drops steeply from z
= 2, reaching the present day value at z ~ 1. In contrast, the SFR density of
intermediate mass galaxies (10^{10.2} < M < 10^{10.8} M_sun) declines more
slowly and may peak or plateau at z ~ 1.5. We use the characteristic growth
time t_SFR = rho_M / rho_SFR to provide evidence of an associated transition in
massive galaxies from a burst to a quiescent star formation mode at z ~ 2.
Intermediate mass systems transit from burst to quiescent mode at z ~ 1, while
the lowest mass objects undergo bursts throughout our redshift range. Our
results show unambiguously that the formation era for galaxies was extended and
proceeded from high to low mass systems. The most massive galaxies formed most
of their stars in the first ~3 Gyr of cosmic history. Intermediate mass objects
continued to form their dominant stellar mass for an additional ~2 Gyr, while
the lowest mass systems have been forming over the whole cosmic epoch spanned
by the GDDS. This view of galaxy formation clearly supports `downsizing' in the
SFR where the most massive galaxies form first and galaxy formation proceeds
from larger to smaller mass scales.Comment: Accepted for publication in ApJ
Gemini Deep Deep Survey VI: Massive Hdelta-strong galaxies at z=1
We show that there has been a dramatic decline in the abundance of massive
galaxies with strong Hdelta stellar absorption lines from z=1.2 to the present.
These ``Hdelta-strong'', or HDS, galaxies have undergone a recent and rapid
break in their star-formation activity. Combining data from the Gemini Deep
Deep and the Sloan Digital Sky Surveys to make mass-matched samples
(M*>=10^10.2 Msun), with 25 and 50,255 galaxies, respectively), we find that
the fraction of galaxies in an HDS phase has decreased from about 50% at z=1.2
to a few percent today. This decrease in fraction is due to an actual decrease
in the number density of massive HDS systems by a factor of 2-4, coupled with
an increase in the number density of massive galaxies by about 30 percent. We
show that this result depends only weakly on the threshold chosen for the
Hdelta equivalent width to define HDS systems (if greater than 4 A) and
corresponds to a (1+z)^{2.5\pm 0.7} evolution. Spectral synthesis studies of
the high-redshift population using the PEGASE code, treating Hdelta_A, EW[OII],
Dn4000, and rest-frame colors, favor models in which the Balmer absorption
features in massive Hdelta-strong systems are the echoes of intense episodes of
star-formation that faded about 1 Gyr prior to the epoch of observation. The
z=1.4-2 epoch appears to correspond to a time at which massive galaxies are in
transition from a mode of sustained star formation to a relatively quiescent
mode with weak and rare star-formation episodes. We argue that the most likely
local descendants of the distant massive HDS galaxies are passively evolving
massive galaxies in the field and small groups.Comment: 16 pages, 12 figures, 3 tables, uses emulateapj.sty; updated to match
the version accepted by ApJ. One figure added, conclusions unchange
The Next Generation Virgo Cluster Survey - Infrared (NGVS-IR): I. A new Near-UV/Optical/Near-IR Globular Cluster selection tool
The NGVS-IR project (Next Generation Virgo Survey - Infrared) is a contiguous
near-infrared imaging survey of the Virgo cluster of galaxies. It complements
the optical wide-field survey of Virgo (NGVS). The current state of NGVS-IR
consists of Ks-band imaging of 4 deg^2 centered on M87, and J and Ks-band
imaging of 16 deg^2 covering the region between M49 and M87. In this paper, we
present the observations of the central 4 deg^2 centered on Virgo's core
region. The data were acquired with WIRCam on the Canada-France-Hawaii
Telescope and the total integration time was 41 hours distributed in 34
contiguous tiles. A survey-specific strategy was designed to account for
extended galaxies while still measuring accurate sky brightness within the
survey area. The average 5\sigma limiting magnitude is Ks=24.4 AB mag and the
50% completeness limit is Ks=23.75 AB mag for point source detections, when
using only images with better than 0.7" seeing (median seeing 0.54"). Star
clusters are marginally resolved in these image stacks, and Virgo galaxies with
\mu_Ks=24.4 AB mag arcsec^-2 are detected. Combining the Ks data with optical
and ultraviolet data, we build the uiK color-color diagram which allows a very
clean color-based selection of globular clusters in Virgo. This diagnostic plot
will provide reliable globular cluster candidates for spectroscopic follow-up
campaigns needed to continue the exploration of Virgo's photometric and
kinematic sub-structures, and will help the design of future searches for
globular clusters in extragalactic systems. Equipped with this powerful new
tool, future NGVS-IR investigations based on the uiK diagram will address the
mapping and analysis of extended structures and compact stellar systems in and
around Virgo galaxies.Comment: 23 pages, 18 figures. Accepted for publication in ApJ
The Gemini Deep Deep Survey: I. Introduction to the Survey, Catalogs and Composite Spectra
The Gemini Deep Deep Survey (GDDS) is an ultra-deep (K<20.6 mag, I<24.5 mag)
redshift survey targeting galaxies in the "redshift desert" between 1<z<2. The
primary goal of the survey is to constrain the space density at high redshift
of evolved high-mass galaxies. We obtained 309 spectra in four widely-separated
30 arcmin^2 fields using the Gemini North telescope and the Gemini Multi-Object
Spectrograph (GMOS). The spectra define a one-in-two sparse sample of the
reddest and most luminous galaxies near the I-K vs. I color-magnitude track
mapped out by passively evolving galaxies in the redshift interval 0.8<z<1.8.
This sample is augmented by a one-in-seven sparse sample of the remaining
high-redshift galaxy population. Typical exposures times were 20-30 hours per
field (in Nod & Shuffle mode), and the resulting spectra are the deepest ever
obtained. In this paper we present our sample of 309 spectra, along with
redshifts, identifications of spectral features, and photometry. The infrared
selection underlying the survey means that the GDDS is observing not only
star-forming galaxies, as in most high-redshift galaxy surveys, but also
quiescent evolved galaxies. The median redshift of the whole GDDS sample is
z=1.1. Together with the data and catalogs, we present a summary of the
criteria for selecting the GDDS fields, the rationale behind our mask designs,
an analysis of the completeness of the survey, and a description of the data
reduction procedures used. All data from the GDDS are publicly available.
(ABRIDGED)Comment: Accepted for publication in AJ. This paper makes very heavy use of
color. A nicer PDF version of this paper with full-resolution figures is
available at the GDDS web site: http://www.ociw.edu/lcirs/gdds.htm
Red Nuggets at z~1.5: Compact passive galaxies and the formation of the Kormendy Relation
We present the results of NICMOS imaging of a sample of 16 high mass
passively evolving galaxies with 1.3<z<2, taken primarily from the Gemini Deep
Deep Survey. Around 80% of galaxies in our sample have spectra dominated by
stars with ages >1 Gyr. Our rest-frame R-band images show that most of these
objects have compact regular morphologies which follow the classical R^1/4 law.
These galaxies scatter along a tight sequence in the Kormendy relation. Around
one-third of the massive red objects are extraordinarily compact, with
effective radii under one kiloparsec. Our NICMOS observations allow the
detection of such systems more robustly than is possible with optical
(rest-frame UV) data, and while similar systems have been seen at z>2, this is
the first time such systems have been detected in a rest-frame optical survey
at 1.3<z<2. We refer to these compact galaxies as "red nuggets". Similarly
compact massive galaxies are completely absent in the nearby Universe. We
introduce a new "stellar mass Kormendy relation" (stellar mass density vs size)
which isolates the effects of size evolution from those of luminosity and color
evolution. The 1.1 < z < 2 passive galaxies have mass densities that are an
order of magnitude larger then early type galaxies today and are comparable to
the compact distant red galaxies at 2 < z < 3. We briefly consider mechanisms
for size evolution in contemporary models focusing on equal-mass mergers and
adiabatic expansion driven by stellar mass loss. Neither of these mechanisms
appears able to transform the high-redshift Kormendy relation into its local
counterpart. Comment: Accepted version (to appear in ApJ
Caltech Faint Galaxy Redshift Survey. XI. The Merger Rate to Redshift 1 from Kinematic Pairs
The rate of mass accumulation due to galaxy merging depends on the mass, density, and velocity distribution of galaxies in the near neighborhood of a host galaxy. The fractional luminosity in kinematic pairs combines all of these effects in a single estimator that is relatively insensitive to population evolution. Here we use a k-corrected and evolution-compensated volume-limited sample having an R-band absolute magnitude of M^(k,e)_R ≤ -19.8 + 5 log h mag drawing about 300 redshifts from the Caltech Faint Galaxy Redshift Survey and 3000 from the Canadian Network for Observational Cosmology field galaxy survey to measure the rate and redshift evolution of merging. The combined sample has an approximately constant comoving number and luminosity density from redshift 0.1 to 1.1 (Ω_M = 0.2, Ω_Λ = 0.8); hence, any merger evolution will be dominated by correlation and velocity evolution, not density evolution. We identify kinematic pairs with projected separations less than either 50 or 100 h^(-1) kpc and rest-frame velocity differences of less than 1000 km s^(-1). The fractional luminosity in pairs is modeled as f_L(Δv, r_p, M^(k,e)_τ)(1 + z)^(m,L), where [f_L, m_L] are [0.14 ± 0.07, 0 ± 1.4] and [0.37 ± 0.7, 0.1 ± 0.5] for r_p ≤ 50 and 100 h^(-1) kpc, respectively (Ω_M = 0.2, Ω_Λ = 0.8). The value of mL is about 0.6 larger if Λ = 0. To convert these redshift-space statistics to a merger rate, we use the data to derive a conversion factor to a physical space pair density, a merger probability, and a mean in-spiral time. The resulting mass accretion rate per galaxy (M_1, M_2 ≥ 0.2M*) is 0.02 ± 0.01(1 + z)^(0.1±0.5)M* Gyr^(-1). Present-day high-luminosity galaxies therefore have accreted approximately 0.15M* of their mass over the approximately 7 Gyr to redshift 1. Since merging is likely only weakly dependent on the host mass, the fractional effect, δM/M 0.15M*/M, is dramatic for lower mass galaxies but is, on the average, effectively perturbative for galaxies above 1M*
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