6,335 research outputs found
A Test of Photometric Redshifts for X-ray Selected Sources
We test the effectiveness of photometric redshifts based upon galaxy spectral
template fitting for X-ray luminous objects, using a sample of 65 sources
detected by Chandra in the field of the Caltech Faint Galaxy Redshift Survey
(CFGRS). We find that sources with quasar-dominated spectra (for which galaxy
spectral templates are not appropriate) are easily identified, and that
photometric redshifts are robust for the rest of the sources in our sample.
Specifically, for the 59 sources that are not quasar-dominated at optical
wavelengths, we find that the photometric redshift estimates have scatter
comparable to the field galaxy population in this region. There is no evidence
for a trend of increasing dispersion with X-ray luminosity over the range L_X =
10^39 - 5x10^43 erg/s, nor is there a trend with the ratio of X-ray to optical
flux, f_X/f_R. The practical implication of this work is that photometric
redshifts should be robust for the majority (~90%) of the X-ray sources down to
f_X ~ 10^-16 erg/s/cm^2 that have optical counterparts brighter than R ~ 24.
Furthermore, the same photometry can be easily used to identify the sources for
which the photometric redshifts are likely to fail. Photometric redshift
estimation can thus be utilized as an efficient tool in analyzing the
statistical properties of upcoming large Chandra and XMM-Newton data sets and
identifying interesting subsamples for further study.Comment: To appear in ApJ (6 pages, 6 figures). Replaced with accepted versio
Distant Galaxy Clusters Identified From Optical Background Fluctuations
We present the first high redshift (0.3 < z < 1.1) galaxy clusters found by
systematically identifying optical low surface brightness fluctuations in the
background sky. Using spectra obtained with the Keck telescope and I-band
images from the Palomar 1.5m telescope, we conclude that at least eight of the
ten candidates examined are high redshift galaxy clusters. The identification
of such clusters from low surface brightness fluctuations provides a
complementary alternative to classic selection methods based on overdensities
of resolved galaxies, and enables us to search efficiently for rich high
redshift clusters over large areas of the sky. The detections described here
are the first in a survey that covers a total of nearly 140 sq. degrees of the
sky and should yield, if these preliminary results are representative, over 300
such clusters.Comment: Submitted to ApJ
Weak lensing mass reconstruction of the interacting cluster 1E0657-558: Direct evidence for the existence of dark matter
We present a weak lensing mass reconstruction of the interacting cluster
1E0657-558 in which we detect both the main cluster and a sub-cluster. The
sub-cluster is identified as a smaller cluster which has just undergone initial
in-fall and pass-through of the primary cluster, and has been previously
identified in both optical surveys and X-ray studies. The X-ray gas has been
separated from the galaxies by ram-pressure stripping during the pass-through.
The detected mass peak is located between the X-ray peak and galaxy
concentration, although the position is consistent with the galaxy centroid
within the errors of the mass reconstruction. We find that the mass peak for
the main cluster is in good spatial agreement with the cluster galaxies and
offset from the X-ray halo at 3.4 sigma significance, and determine that the
mass-to-light ratios of the two components are consistent with those of relaxed
clusters. The observed offsets of the lensing mass peaks from the peaks of the
dominant visible mass component (the X-ray gas) directly demonstrate the
presence, and dominance, of dark matter in this cluster. This proof of the dark
matter existence holds true even under the assumption of modified Newtonian
gravity (MOND); from the observed gravitational shear to optical light ratios
and mass peak - X-ray gas offsets, the dark matter component in a MOND regime
has a total mass which is at least equal to the baryonic mass of the system.Comment: 8 pages, 4 figure, accepted by Ap
Forming Early-type Galaxies in Groups Prior to Cluster Assembly
We study a unique proto-cluster of galaxies, the supergroup SG1120-1202. We
quantify the degree to which morphological transformation of cluster galaxies
occurs prior to cluster assembly in order to explain the observed early-type
fractions in galaxy clusters at z=0. SG1120-1202 at z~0.37 is comprised of four
gravitationally bound groups that are expected to coalesce into a single
cluster by z=0. Using HST ACS observations, we compare the morphological
fractions of the supergroup galaxies to those found in a range of environments.
We find that the morphological fractions of early-type galaxies (~60 %) and the
ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to
clusters at comparable redshift, consistent with pre-processing in the group
environment playing the dominant role in establishing the observed early-type
fraction in galaxy clusters.Comment: 5 pages, 2 figures, 2 tables. Accepted for publication in ApJ Letter
Tests of the Las Campanas Distant Cluster Survey from Confirmation Observations for the ESO Distant Cluster Survey
The ESO Distant Cluster Survey (EDisCS) is a photometric and spectroscopic
study of the galaxy cluster population at two epochs, z~0.5 and z~0.8, drawn
from the Las Campanas Distant Cluster Survey (LCDCS). We report results from
the initial candidate confirmation stage of the program and use these results
to probe the properties of the LCDCS. Of the 30 candidates targeted, we find
statistically significant overdensities of red galaxies near 28. Of the ten
additional candidates serendipitously observed within the fields of the
targeted 30, we detect red galaxy overdensities near six. We test the
robustness of the published LCDCS estimated redshifts to misidentification of
the brighest cluster galaxy (BCG) in the survey data, and measure the spatial
alignment of the published cluster coordinates, the peak red galaxy
overdensity, and the brightest cluster galaxy. We conclude that for LCDCS
clusters out to z~0.8, 1) the LCDCS coordinates agree with the centroid of the
red galaxy overdensity to within 25'' (~150 h^{-1} kpc) for 34 out of 37
candidates with 3\sigma galaxy overdensities, 2) BCGs are typically coincident
with the centroid of the red galaxy population to within a projected separation
of 200 h^{-1} kpc (32 out of 34 confirmed candidates), 3) the red galaxy
population is strongly concentrated, and 4) the misidentification of the BCG in
the LCDCS causes a redshift error >0.1 in 15-20% of the LCDCS candidates. These
findings together help explain the success of the surface brightness
fluctuations detection method.Comment: 10 pages, 9 figures, accepted for publication in the November 10
issue of Ap
Evidence for a constant IMF in early-type galaxies based on their X-ray binary populations
A number of recent studies have proposed that the stellar initial mass
function (IMF) of early type galaxies varies systematically as a function of
galaxy mass, with higher mass galaxies having bottom heavy IMFs. These bottom
heavy IMFs have more low-mass stars relative to the number of high mass stars,
and therefore naturally result in proportionally fewer neutron stars and black
holes. In this paper, we specifically predict the variation in the number of
black holes and neutron stars based on the power-law IMF variation required to
reproduce the observed mass-to-light ratio trends with galaxy mass. We then
test whether such variations are observed by studying the field low-mass X-ray
binary populations (LMXBs) of nearby early-type galaxies. In these binaries, a
neutron star or black hole accretes matter from a low-mass donor star. Their
number is therefore expected to scale with the number of black holes and
neutron stars present in a galaxy. We find that the number of LMXBs per K-band
light is similar among the galaxies in our sample. These data therefore
demonstrate the uniformity of the slope of the IMF from massive stars down to
those now dominating the K-band light, and are consistent with an invariant
IMF. Our results are inconsistent with an IMF which varies from a
Kroupa/Chabrier like IMF for low mass galaxies to a steep power-law IMF (with
slope =2.8) for high mass galaxies. We discuss how these observations
constrain the possible forms of the IMF variations and how future Chandra
observations can enable sharper tests of the IMF.Comment: 12 pages, 5 figures, 2 tables, submitted to Ap
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