543 research outputs found
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
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
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
RXJ0848.6+4453: The Evolution of Galaxy Sizes and Stellar Populations in a z=1.27 Cluster
RXJ0848.6+4453 (Lynx W) at redshift 1.27 is part of the Lynx Supercluster of
galaxies. Our analysis of stellar populations and star formation history in the
cluster covers 24 members and is based on deep optical spectroscopy from Gemini
North and imaging data from HST. Focusing on the 13 bulge-dominated galaxies
for which we can determine central velocity dispersions, we find that these
show a smaller evolution of sizes and velocity dispersions than reported for
field galaxies and galaxies in poorer clusters. The galaxies in RXJ0848.6+4453
populate the Fundamental Plane similar to that found for lower redshift
clusters with a zero point offset corresponding to an epoch of last star
formation at z_form= 1.95+-0.2. The spectra of the galaxies in RXJ0848.6+4453
are dominated by young stellar populations at all galaxy masses and in many
cases show emission indicating low level on-going star formation. The average
age of the young stellar populations (estimated from H-zeta) is consistent with
a major star formation episode 1-2 Gyr prior, which in turn agrees with
z_form=1.95. Galaxies dominated by young stellar populations are distributed
throughout the cluster. We speculate that low level star formation has not yet
been fully quenched in the center of this cluster may be because the cluster is
significantly poorer than other clusters previously studied at similar
redshifts, which appear to have very little on-going star formation in their
centers.Comment: Accepted for publication in Astronomical Journal. High-resolution
figures available from the first author by reques
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