721 research outputs found
The Detection of a Red Sequence of Massive Field Galaxies at z~2.3 and its Evolution to z~0
The existence of massive galaxies with strongly suppressed star formation at
z~2.3, identified in a previous paper, suggests that a red sequence may already
be in place beyond z=2. In order to test this hypothesis, we study the
rest-frame U-B color distribution of massive galaxies at 2<z<3. The sample is
drawn from our near-infrared spectroscopic survey for massive galaxies. The
color distribution shows a statistically significant (>3 sigma) red sequence,
which hosts ~60% of the stellar mass at the high-mass end. The red-sequence
galaxies have little or no ongoing star formation, as inferred from both
emission-line diagnostics and stellar continuum shapes. Their strong Balmer
breaks and their location in the rest-frame U-B, B-V plane indicate that they
are in a post-starburst phase, with typical ages of ~0.5-1.0 Gyr. In order to
study the evolution of the red sequence, we compare our sample with
spectroscopic massive galaxy samples at 0.02<z<0.045 and 0.6<z<1.0. The
rest-frame U-B color reddens by ~0.25 mag from z~2.3 to the present at a given
mass. Over the same redshift interval, the number and stellar mass density on
the high-mass end (>10^11 Msol) of the red sequence grow by factors of ~8 and
~6, respectively. We explore simple models to explain the observed evolution.
Passive evolution models predict too strong d(U-B), and produce z~0 galaxies
that are too red. More complicated models that include aging, galaxy
transformations, and red mergers can explain both the number density and color
evolution of the massive end of the red sequence between z~2.3 and the present.Comment: Accepted for publication in the Astrophysical Journa
The Evolution of the Field and Cluster Morphology-Density Relation for Mass-Selected Samples of Galaxies
The Sloan Digital Sky Survey (SDSS) and photometric/spectroscopic surveys in
the GOODS-South field (the Chandra Deep Field-South, CDFS) are used to
construct volume-limited, stellar mass-selected samples of galaxies at
redshifts 0<z<1. The CDFS sample at 0.6<z<1.0 contains 207 galaxies complete
down to M=4x10^10 Msol (for a ``diet'' Salpeter IMF), corresponding to a
luminosity limit for red galaxies of M_B=-20.1. The SDSS sample at
0.020<z<0.045 contains 2003 galaxies down to the same mass limit, which
corresponds to M_B=-19.3 for red galaxies. Morphologies are determined with an
automated method, using the Sersic parameter n and a measure of the residual
from the model fits, called ``bumpiness'', to distinguish different
morphologies. These classifications are verified with visual classifications.
In agreement with previous studies, 65-70% of the galaxies are located on the
red sequence, both at z~0.03 and at z~0.8. Similarly, 65-70% of the galaxies
have n>2.5. The fraction of E+S0 galaxies is 43+/-3%$ at z~0.03 and 48+/-7% at
z~0.8, i.e., it has not changed significantly since z~0.8. When combined with
recent results for cluster galaxies in the same redshift range, we find that
the morphology-density relation for galaxies more massive than 0.5M* has
remained constant since at least z~0.8. This implies that galaxies evolve in
mass, morphology and density such that the morphology-density relation does not
change. In particular, the decline of star formation activity and the
accompanying increase in the stellar mass density of red galaxies since z~1
must happen without large changes in the early-type galaxy fraction in a given
environment.Comment: 16 pages, 13 figures, 2 tables. Updated to match journal version.
Will appear in ApJ (vol. 670, p. 206
The Nascent Red Sequence at z~2
We present new constraints on the evolution of the early-type galaxy
color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy
protocluster at z=2.16 obtained using NICMOS on-board the Hubble Space
Telescope. This field contains a spectroscopically confirmed space-overdensity
of Lyman-alpha and H-alpha emitting galaxies which surrounds the powerful radio
galaxy MRC 1138-262. Using these NICMOS data we identify a significant
surface-overdensity (= 6.2x) of red J-H galaxies in the color-magnitude diagram
(when compared with deep NICMOS imaging from the HDF-N and UDF). The
optical-NIR colors of these prospective red-sequence galaxies indicate the
presence of on-going dust-obscured star-formation or recently formed (<~ 1.5
Gyr)stellar populations in a majority of the red galaxies. We measure the slope
and intrinsic scatter of the CMR for three different red galaxy samples
selected by a wide color cut, and using photometric redshifts both with and
without restrictions on rest-frame optical morphology. In all three cases both
the rest-frame slope and intrinsic color scatter are considerably higher
than corresponding values for lower redshift galaxy clusters. These results
suggest that while some relatively quiescent galaxies do exist in this
protocluster both the majority of the galaxy population and hence the
color-magnitude relation are still in the process of forming, as expected.Comment: 8 pages, 7 figures, accepted for publication in ApJ (to appear June
1, 2008, v679n2
Discovery of a Ringlike Dark Matter Structure in the Core of the Galaxy Cluster Cl 0024+17
We present a comprehensive mass reconstruction of the rich galaxy cluster Cl
0024+17 at z~0.4 from ACS data, unifying both strong- and weak-lensing
constraints. The weak-lensing signal from a dense distribution of background
galaxies (~120 per square arcmin) across the cluster enables the derivation of
a high-resolution parameter-free mass map. The strongly-lensed objects tightly
constrain the mass structure of the cluster inner region on an absolute scale,
breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17
obtained in such a way is remarkable. It reveals a ringlike dark matter
substructure at r~75" surrounding a soft, dense core at r~50". We interpret
this peculiar sub-structure as the result of a high-speed line-of-sight
collision of two massive clusters 1-2 Gyr ago. Such an event is also indicated
by the cluster velocity distribution. Our numerical simulation with purely
collisionless particles demonstrates that such density ripples can arise by
radially expanding, decelerating particles that originally comprised the
pre-collision cores. Cl 0024+17 can be likened to the bullet cluster 1E0657-56,
but viewed the collision axis at a much later epoch. In addition, we
show that the long-standing mass discrepancy for Cl 0024+17 between X-ray and
lensing can be resolved by treating the cluster X-ray emission as coming from a
superposition of two X-ray systems. The cluster's unusual X-ray surface
brightness profile that requires a two isothermal sphere description supports
this hypothesis.Comment: To appear in the June 1 issue of The Astrophysical Journa
CANDELS Observations of the Structural Properties and Evolution of Galaxies in a Cluster at z=1.62
We discuss the structural and morphological properties of galaxies in a
z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope
Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology
relation: galaxies with colors of quiescent stellar populations generally have
morphologies consistent with spheroids, and galaxies with colors consistent
with ongoing star formation have disk-like and irregular morphologies. The size
distribution of the quiescent cluster galaxies shows a deficit of compact (<
1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a
result the cluster quiescent galaxies have larger average effective sizes
compared to field galaxies at fixed mass at greater than 90% significance.
Combined with data from the literature, the size evolution of quiescent cluster
galaxies is relatively slow from z~1.6 to the present, growing as
(1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that
physical processes associated with the denser cluster region seems to have
caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower
subsequent growth at z<1.6 compared to galaxies in the lower density field. The
quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower
redshift samples at fixed mass, and their surface-brightness profiles suggest
that they contain extended stellar disks. We argue the cluster galaxies require
dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk
material and to match the relations for ellipticity, stellar mass, size, and
color of early-type galaxies in z<1 clusters.Comment: Accepted for publication in ApJ. 14 pages in emulateapj format.
Replacement includes improvements from referee report, and updates and
additions to reference
Geometry of Star-Forming Galaxies from SDSS, 3D-HST and CANDELS
We determine the intrinsic, 3-dimensional shape distribution of star-forming
galaxies at 0<z<2.5, as inferred from their observed projected axis ratios. In
the present-day universe star-forming galaxies of all masses 1e9 - 1e11 Msol
are predominantly thin, nearly oblate disks, in line with previous studies. We
now extend this to higher redshifts, and find that among massive galaxies (M* >
1e10 Msol) disks are the most common geometric shape at all z < 2. Lower-mass
galaxies at z>1 possess a broad range of geometric shapes: the fraction of
elongated (prolate) galaxies increases toward higher redshifts and lower
masses. Galaxies with stellar mass 1e9 Msol (1e10 Msol) are a mix of roughly
equal numbers of elongated and disk galaxies at z~1 (z~2). This suggests that
galaxies in this mass range do not yet have disks that are sustained over many
orbital periods, implying that galaxies with present-day stellar mass
comparable to that of the Milky Way typically first formed such sustained
stellar disks at redshift z~1.5-2. Combined with constraints on the evolution
of the star formation rate density and the distribution of star formation over
galaxies with different masses, our findings imply that, averaged over cosmic
time, the majority of stars formed in disks.Comment: Published in ApJ Letter
The merger-driven evolution of massive galaxies
We explore the rate and impact of galaxy mergers on the massive galaxy
population using the amplitude of the two-point correlation function on small
scales for M > 5e10 M_sun galaxies from the COSMOS and COMBO-17 surveys. Using
a pair fraction derived from the Sloan Digital Sky Survey as a low-redshift
benchmark, the large survey area at intermediate redshifts allows us to
determine the evolution of the close pair fraction with unprecedented accuracy
for a mass-selected sample: we find that the fraction of galaxies more massive
than 5e10M_sun in pairs separated by less than 30 kpc in 3D space evolves as
F(z) = (0.0130+/-0.0019)x(1+z)^1.21+/-0.25 between z = 0 and z = 1.2. Assuming
a merger time scale of 0.5 Gyrs, the inferred merger rate is such that galaxies
with mass in excess of 1e11 M_sun have undergone, on average, 0.5 (0.7) mergers
involving progenitor galaxies both more massive than 5e10 M_sun since z = 0.6
(1.2). We also study the number density evolution of massive red sequence
galaxies using published luminosity functions and constraints on the M/L
evolution from the fundamental plane. Moreover, we demonstrate that the
measured merger rate of massive galaxies is sufficient to explain this observed
number density evolution in massive red sequence galaxies since z = 1.Comment: Accepted in Ap
What turns galaxies off? The different morphologies of star-forming and quiescent galaxies since z~2 from CANDELS
We use HST/WFC3 imaging from the CANDELS Multicycle Treasury Survey, in
conjunction with the Sloan Digital Sky Survey, to explore the evolution of
galactic structure for galaxies with stellar masses >3e10M_sun from z=2.2 to
the present epoch, a time span of 10Gyr. We explore the relationship between
rest-frame optical color, stellar mass, star formation activity and galaxy
structure. We confirm the dramatic increase from z=2.2 to the present day in
the number density of non-star-forming galaxies above 3e10M_sun reported by
others. We further find that the vast majority of these quiescent systems have
concentrated light profiles, as parametrized by the Sersic index, and the
population of concentrated galaxies grows similarly rapidly. We examine the
joint distribution of star formation activity, Sersic index, stellar mass,
inferred velocity dispersion, and stellar surface density. Quiescence
correlates poorly with stellar mass at all z<2.2. Quiescence correlates well
with Sersic index at all redshifts. Quiescence correlates well with `velocity
dispersion' and stellar surface density at z>1.3, and somewhat less well at
lower redshifts. Yet, there is significant scatter between quiescence and
galaxy structure: while the vast majority of quiescent galaxies have prominent
bulges, many of them have significant disks, and a number of bulge-dominated
galaxies have significant star formation. Noting the rarity of quiescent
galaxies without prominent bulges, we argue that a prominent bulge (and
perhaps, by association, a supermassive black hole) is an important condition
for quenching star formation on galactic scales over the last 10Gyr, in
qualitative agreement with the AGN feedback paradigm.Comment: The Astrophysical Journal, in press; 20 pages with 13 figure
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