34 research outputs found
Galaxy pairs as a probe for mergers at z ~ 2
In this work I investigate the redshift evolution of pair fraction of a
sample of 196 massive galaxies from z = 0 to 3, selected from the COSMOS field.
We find that on average a massive galaxy undergoes ~ 1.1 \pm 0.5 major merger
since z = 3. I will review the current limitations of using the pair fraction
as a probe for quantifying the impact of mergers on galaxy evolution. This work
is based on the paper Man et al. (2011).Comment: 4 pages; to appear on the Conference Proceedings for "Galaxy Mergers
in an Evolving Universe", held in Hualien, Taiwan (October 2011
Evidence for Widespread AGN Activity among Massive Quiescent Galaxies at z ~ 2
We quantify the presence of Active Galactic nuclei (AGN) in a mass-complete
(M_* >5e10 M_sun) sample of 123 star-forming and quiescent galaxies at 1.5 < z
< 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey.
41+/-7% of the galaxies are detected directly in X-rays, 22+/-5% with
rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGN
(L_0.5-8keV > 3e42 ergs/s). The latter fraction is similar for star-forming and
quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that
perhaps luminous AGN are triggered by external effects such as mergers. We
detect significant mean X-ray signals in stacked images for both the
individually non-detected star-forming and quiescent galaxies, with spectra
consistent with star formation only and/or a low luminosity AGN in both cases.
Comparing star formation rates inferred from the 2-10 keV luminosities to those
from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative
of low-luminosity AGN. Among the quiescent galaxies, the excess suggests that
as many as 70-100% of these contain low- or high-luminosity AGN, while the
corresponding fraction is lower among star-forming galaxies (43-65%). The
ubiquitous presence of AGN in massive, quiescent z ~ 2 galaxies that we find
provides observational support for the importance of AGN in impeding star
formation during galaxy evolution.Comment: 9 pages, 3 figures, 3 tables; Accepted for publication in ApJ. Minor
editing changes and a few references added. Matches published versio
Evolution of the Lyman-alpha Halos around High-Redshift Radio Galaxies
We have obtained the first constraints on extended Ly-alpha emission at z ~ 1
in a sample of five radio galaxies. We detect Ly-alpha emission from four of
the five galaxies. The Ly-alpha luminosities range from 0.1 - 4 times 10^43
erg/s and are much smaller than those observed for halos around higher redshift
radio galaxies. If the z ~ 1 radio galaxies are the descendents the z >~ 2
radio galaxies, then their Ly-alpha luminosities evolve strongly with redshift
as ~(1+z)^5. There do not appear to be strong correlations between other
parameters, such as radio power, suggesting that this observed evolution is
real and not an observational artifact or secondary correlation. We speculate
that this evolution of luminous halos may be due to gas depletion (as gas
cools, settles, and forms stars) accompanied by an overall rise in the mean gas
temperature and a decrease in specific star-formation rate in and around these
massive galaxies.Comment: 5 pages, 4 figures, published in ApJ Letters, 694, L31-35 March 20
200
A spectroscopic sample of massive, evolved z~2 galaxies: Implications for the evolution of the mass-size relation
We present deep, near-infrared HST/WFC3 grism spectroscopy and imaging for a
sample of 14 galaxies at z~2 selected from a mass-complete photometric catalog
in the COSMOS field. By combining the grism observations with photometry in 30
bands, we derive accurate constraints on their redshifts, stellar masses, ages,
dust extinction and formation redshifts. We show that the slope and scatter of
the z~2 mass-size relation of quiescent galaxies is consistent with the local
relation, and confirm previous findings that the sizes for a given mass are
smaller by a factor of two to three. Finally, we show that the observed
evolution of the mass-size relation of quiescent galaxies between z=2 and 0 can
be explained by quenching of increasingly larger star-forming galaxies, at a
rate dictated by the increase in the number density of quiescent galaxies with
decreasing redshift. However, we find that the scatter in the mass-size
relation should increase in the quenching-driven scenario in contrast to what
is seen in the data. This suggests that merging is not needed to explain the
evolution of the median mass-size relation of massive galaxies, but may still
be required to tighten its scatter, and explain the size growth of individual
z=2 galaxies quiescent galaxies.Comment: 16 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Structure of Protocluster Galaxies: Accelerated Structural Evolution in Overdense Environments?
We present a high spatial-resolution HST/NICMOS imaging survey in the field
of a known protocluster surrounding the powerful radio galaxy MRC1138-262 at
z=2.16. Previously, we have shown that this field exhibits a substantial
surface overdensity of red J-H galaxies. Here we focus on the stellar masses
and galaxy effective radii in an effort to compare and contrast the properties
of likely protocluster galaxies with their field counterparts and to look for
correlations between galaxy structure and (projected) distance relative to the
radio galaxy.
We find a hint that quiescent, cluster galaxies are on average less dense
than quiescent field galaxies of similar stellar mass and redshift. In fact, we
find only two (of nine) quiescent protocluster galaxies are of simliar density
to the majority of the massive, quiescent compact galaxies (SEEDs) found in
several field surveys. Furthermore, there is some indication that the
structural Sersic n parameter is higher (n ~ 3-4) on average for cluster
galaxies compared to the field SEEDs (n ~ 1-2) This result may imply that the
accelerated galaxy evolution expected (and observed) in overdense regions also
extends to structural evolution presuming that massive galaxies began as dense
(low n) SEEDs and have already evolved to be more in line with local galaxies
of the same stellar mass.Comment: 11 pages, 7 figures, 1 table, Accepted for publication in Ap
The pair faction of massive galaxies at 0 ≤ z ≤ 3
Using a mass-selected (M-star >= 10(11) M-circle dot) sample of 198 galaxies at 0 <= z <= 3.0 with Hubble Space Telescope/NICMOS H-160-band images from the COSMOS survey, we find evidence for the evolution of the pair fraction above z similar to 2, an epoch in which massive galaxies are believed to undergo significant structural and mass evolution. We observe that the pair fraction of massive galaxies is 0.15 +/- 0.08 at 1.7 <= z <= 3.0, where galaxy pairs are defined as massive galaxies having a companion of flux ratio from 1:1 to 1:4 within a projected separation of 30 kpc. This is slightly lower but still consistent with the pair fraction measured previously in other studies, and the merger fraction predicted in halo-occupation modeling. The redshift evolution of the pair fraction is described by a power law F(z) = (0.07 +/- 0.04) x (1 + z)(0.6 +/- 0.5). The merger rate is consistent with no redshift evolution; however it is difficult to constrain due to the limited sample size and the high uncertainties in the merging timescale. Based on the merger rate calculation, we estimate that a massive galaxy undergoes on average 1.1 +/- 0.5 major mergers from z = 3 to 0. The observed merger fraction is sufficient to explain the number density evolution of massive galaxies, but insufficient to explain the size evolution. This is a hint that mechanism(s) other than major merging may be required to increase the sizes of the massive, compact quiescent galaxies from z similar to 2 to 0
Recent structural evolution of early-type galaxies : size growth from z=1 to z=0
Strong size and internal density evolution of early-type galaxies between z similar to 2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z similar to 1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion (sigma) measurements are taken from the literature for 50 morphologically selected 0.8 < z < 1.2 field and cluster early-type galaxies with typical masses M-dyn = 2 x 10(11) M-circle dot. Sizes (R-eff) are determined with Advanced Camera for Surveys imaging. We compare the distant sample with a large sample of nearby (0.04 < z < 0.08) early-type galaxies extracted from the Sloan Digital Sky Survey for which we determine sizes, masses, and densities in a consistent manner, using simulations to quantify systematic differences between the size measurements of nearby and distant galaxies. We find a highly significant difference between the sigma-R-eff distributions of the nearby and distant samples, regardless of sample selection effects. The implied evolution in R-eff at fixed mass between z = 1 and the present is a factor of 1.97 +/- 0.15. This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since z similar to 2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations
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
Massive Ellipticals at High Redshift: NICMOS Imaging of Z~1 Radio Galaxies
We present deep, continuum images of eleven high-redshift (0.811 < z < 1.875)
3CR radio galaxies observed with NICMOS. Our images probe the rest-frame
optical light where stars are expected to dominate the galaxy luminosity. The
rest-frame UV light of eight of these galaxies demonstrates the well-known
``alignment effect''. Most of the radio galaxies have rounder, more symmetric
morphologies at rest-frame optical wavelengths. Here we show the most direct
evidence that in most cases the stellar hosts are normal elliptical galaxies
with de Vaucouleurs law light profiles. For a few galaxies very faint traces of
the UV-bright aligned component are also visible in the infrared images. We
derive both the effective radius and surface-brightness for nine of eleven
sample galaxies by fitting surface-brightness models to them. We find their
sizes are similar to those of local FRII radio source hosts and are in general
larger than other local galaxies. The derived host galaxy luminosities are very
high and lie at the bright end of luminosity functions constructed at similar
redshifts. The galaxies in our sample are also brighter than the rest-frame
size--surface-brightness locus defined by the low-redshift sources. Passive
evolution roughly aligns the z ~ 1 galaxies with the low-redshift samples. The
optical host is sometimes centered on a local minimum in the rest-frame UV
emission, suggesting the presence of substantial dust obscuration. We also see
good evidence of nuclear point sources in three galaxies. Overall, our results
are consistent with the hypothesis that these galaxies have already formed the
bulk of their stars at redshifts greater than z >~ 2, and that the AGN
phenomenon takes place within otherwise normal, perhaps passively evolving,
galaxies. (abridged)Comment: 28 pages, 14 figures, accepted to ApJ. Uses AASTEX and emulateapj