111 research outputs found
Pair Analysis of Field Galaxies from the Red-Sequence Cluster Survey
We study the evolution of the number of close companions of similar
luminosities per galaxy (Nc) by choosing a volume-limited subset of the
photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The
sample contains over 157,000 objects with a moderate redshift range of 0.25 < z
< 0.8 and absolute magnitude in Rc (M_Rc) < -20. This is the largest sample
used for pair evolution analysis, providing data over 9 redshift bins with
about 17,500 galaxies in each. After applying incompleteness and projection
corrections, Nc shows a clear evolution with redshift. The Nc value for the
whole sample grows with redshift as (1+z)^m, where m = 2.83 +/- 0.33 in good
agreement with N-body simulations in a LCDM cosmology. We also separate the
sample into two different absolute magnitude bins: -25 < M_Rc < -21 and -21 <
M_Rc < -20, and find that the brighter the absolute magnitude, the smaller the
m value. Furthermore, we study the evolution of the pair fraction for different
projected separation bins and different luminosities. We find that the m value
becomes smaller for larger separation, and the pair fraction for the fainter
luminosity bin has stronger evolution. We derive the major merger remnant
fraction f_rem = 0.06, which implies that about 6% of galaxies with -25 < M_Rc
< -20 have undergone major mergers since z = 0.8.Comment: ApJ, in pres
Stellar Populations of Highly Magnified Lensed Galaxies: Young Starbursts at z~2
We present a comprehensive analysis of the rest-frame UV to near-IR spectral
energy distributions and rest-frame optical spectra of four of the brightest
gravitationally lensed galaxies in the literature: RCSGA 032727-132609 at
z=1.70, MS1512-cB58 at z=2.73, SGAS J152745.1+065219 at z=2.76 and SGAS
J122651.3+215220 at z=2.92. This includes new Spitzer imaging for RCSGA0327 as
well as new spectra, near-IR imaging and Spitzer imaging for SGAS1527 and
SGAS1226. Lensing magnifications of 3-4 magnitudes allow a detailed study of
the stellar populations and physical conditions. We compare star formation
rates as measured from the SED fit, the H-alpha and [OII] emission lines, and
the UV+IR bolometric luminosity where 24 micron photometry is available. The
SFR estimate from the SED fit is consistently higher than the other indicators,
which suggests that the Calzetti dust extinction law used in the SED fitting is
too flat for young star-forming galaxies at z~2. Our analysis finds similar
stellar population parameters for all four lensed galaxies: stellar masses
3-7*10^9 M_sun, young ages ~ 100 Myr, little dust content E(B-V)=0.10-0.25, and
star formation rates around 20-100 M_sun/yr. Compared to typical values for the
galaxy population at z~2, this suggests we are looking at newly formed,
starbursting systems that have only recently started the build-up of stellar
mass. These results constitute the first detailed, uniform analysis of a sample
of the growing number of strongly lensed galaxies known at z~2.Comment: 13 pages, 8 figures, Accepted to Ap
A z=0.9 supercluster of X-ray luminous, optically-selected, massive galaxy clusters
We report the discovery of a compact supercluster structure at z=0.9. The
structure comprises three optically-selected clusters, all of which are
detected in X-rays and spectroscopically confirmed to lie at the same redshift.
The Chandra X-ray temperatures imply individual masses of ~5x10^14 Msun. The
X-ray masses are consistent with those inferred from optical--X-ray scaling
relations established at lower redshift. A strongly-lensed z~4 Lyman break
galaxy behind one of the clusters allows a strong-lensing mass to be estimated
for this cluster, which is in good agreement with the X-ray measurement.
Optical spectroscopy of this cluster gives a dynamical mass in good agreement
with the other independent mass estimates. The three components of the
RCS2319+00 supercluster are separated from their nearest neighbor by a mere <3
Mpc in the plane of the sky and likely <10 Mpc along the line-of-sight, and we
interpret this structure as the high-redshift antecedent of massive (~10^15
Msun) z~0.5 clusters such as MS0451.5-0305.Comment: ApJ Letters accepted. 5 pages in emulateapj, 3 figure
Spectroscopy of moderately high-redshift RCS-1 clusters
We present spectroscopic observations of 11 moderately high-redshift (z~0.7-
1.0) clusters from the first Red-Sequence Cluster Survey (RCS-1). We find
excellent agreement between the red-sequence estimated redshift and the
spectroscopic redshift, with a scatter of 10% at z>0.7. At the high-redshift
end (z>~0.9) of the sample, we find two of the systems selected are projections
of pairs of comparably rich systems, with red-sequences too close to
discriminate in (R-z') colour. In one of these systems, the two components are
close enough to be physically associated. For a subsample of clusters with
sufficient spectroscopic members, we examine the correlation between B_gcR
(optical richness) and the dynamical mass inferred from the velocity
dispersion. We find these measurements to be compatible, within the relatively
large uncertainties, with the correlation established at lower redshift for the
X-ray selected CNOC1 clusters and also for a lower redshift sample of RCS-1
clusters. Confirmation of this and calibration of the scatter in the relation
will require larger samples of clusters at these and higher redshifts.
[abridged]Comment: AJ accepted. 30 pages, 7 figures (figure 5 reduced quality
Galaxy Clusters in the Line of Sight to Background Quasars: I. Survey Design and Incidence of MgII Absorbers at Cluster Redshifts
We describe the first optical survey of absorption systems associated with
galaxy clusters at z= 0.3-0.9. We have cross-correlated SDSS DR3 quasars with
high-redshift cluster/group candidates from the Red-Sequence Cluster Survey. We
have found 442 quasar-cluster pairs for which the MgII doublet might be
detected at a transverse (physical) distance d<2 Mpc from the cluster centers.
To investigate the incidence (dN/dz) and equivalent-width distribution n(W) of
MgII systems at cluster redshifts, two statistical samples were drawn out of
these pairs: one made of high-resolution spectroscopic quasar observations (46
pairs), and one made of quasars used in MgII searches found in the literature
(375 pairs). The results are: (1) the population of strong MgII systems
(W_0>2.0 Ang.) near cluster redshifts shows a significant (>3 sigma)
overabundance (up to a factor of 15) when compared with the 'field' population;
(2) the overabundance is more evident at smaller distances (d<1 Mpc) than
larger distances (d<2 Mpc) from the cluster center; and, (3) the population of
weak MgII systems (W_0<0.3 Ang.) near cluster redshifts conform to the field
statistics. Unlike in the field, this dichotomy makes n(W) in clusters appear
flat and well fitted by a power-law in the entire W-range. A sub-sample of the
most massive clusters yields a stronger and still significant signal. Since
either the absorber number density or filling-factor/cross-section affects the
absorber statistics, an interesting possibility is that we have detected the
signature of truncated halos due to environmental effects. Thus, we argue that
the excess of strong systems is due to a population of absorbers in an
overdense galaxy environment, and the lack of weak systems to a different
population, that got destroyed in the cluster environment. (Abridged)Comment: Accepted for publication in the Astrophysical Journa
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