3,348 research outputs found
Galactic extinction and Abell clusters
In this paper, we present the results of comparing the angular distribution
of Abell clusters with Galactic HI measurements. For most subsamples of
clusters considered, their positions on the sky appear to be anti-correlated
with respect to the distribution of HI column densities. The statistical
significance of these observed anti-correlations is a function of both richness
and distance class, with the more distant and/or richest systems having the
highest significance (~3 sigma). The lower richness, nearby clusters appear to
be randomly distributed compared to the observed Galactic HI column density.Comment: 5 pages, uuencoded compressed postscript file. Figures included.
Accepted by MNRA
A Bayesian Inference Analysis of the X-ray Cluster Luminosity-Temperature Relation
We present a Bayesian inference analysis of the Markevitch (1998) and Allen &
Fabian (1998) cooling flow corrected X-ray cluster temperature catalogs that
constrains the slope and the evolution of the empirical X-ray cluster
luminosity-temperature (L-T) relation. We find that for the luminosity range
10^44.5 erg s^-1 < L_bol < 10^46.5 erg s^-1 and the redshift range z < 0.5,
L_bol is proportional to T^2.80(+0.15/-0.15)(1+z)^(0.91-1.12q_0)(+0.54/-1.22).
We also determine the L-T relation that one should use when fitting the Press-
Schechter mass function to X-ray cluster luminosity catalogs such as the
Einstein Medium Sensitivity Survey (EMSS) and the Southern Serendipitous High-
Redshift Archival ROSAT Catalog (Southern SHARC), for which cooling flow
corrected luminosities are not determined and a universal X-ray cluster
temperature of T = 6 keV is assumed. In this case, L_bol is proportional to
T^2.65(+0.23/-0.20)(1+z)^(0.42-1.26q_0)(+0.75/-0.83) for the same luminosity
and redshift ranges.Comment: Accepted to The Astrophysical Journal, 20 pages, LaTe
The Las Campanas Distant Cluster Survey -- The Correlation Function
We present the first non-local (z>0.2) measurement of the cluster-cluster
spatial correlation length, using data from the Las Campanas Distant Cluster
Survey (LCDCS). We measure the angular correlation function for
velocity-dispersion limited subsamples of the catalog at estimated redshifts of
0.35<z_{est}<0.575, and derive spatial correlation lengths for these clusters
via the cosmological Limber equation. The correlation lengths that we measure
for clusters in the LCDCS are consistent both with local results for the APM
cluster catalog and with theoretical expectations based upon the Virgo
Consortium Hubble Volume simulations and the analytic predictions. Despite
samples containing over 100 clusters, our ability to discriminate between
cosmological models is limited because of statistical uncertainty.Comment: 7 pages, 4 figures, accepted to ApJ (v571, May 20, 2002
An Isocurvature CDM Cosmogony. II. Observational Tests
A companion paper presents a worked model for evolution through inflation to
initial conditions for an isocurvature model for structure formation. It is
shown here that the model is consistent with the available observational
constraints that can be applied without the help of numerical simulations. The
model gives an acceptable fit to the second moments of the angular fluctuations
in the thermal background radiation and the second through fourth moments of
the measured large-scale fluctuations in galaxy counts, within the possibly
significant uncertainties in these measurements. The cluster mass function
requires a rather low but observationally acceptable mass density,
0.1\lsim\Omega\lsim 0.2 in a cosmologically flat universe. Galaxies would be
assembled earlier in this model than in the adiabatic version, an arguably good
thing. Aspects of the predicted non-Gaussian character of the anisotropy of the
thermal background radiation in this model are discussed.Comment: 14 pages, 3 postscript figures, uses aas2pp4.st
A new source detection algorithm using FDR
The False Discovery Rate (FDR) method has recently been described by Miller
et al (2001), along with several examples of astrophysical applications. FDR is
a new statistical procedure due to Benjamini and Hochberg (1995) for
controlling the fraction of false positives when performing multiple hypothesis
testing. The importance of this method to source detection algorithms is
immediately clear. To explore the possibilities offered we have developed a new
task for performing source detection in radio-telescope images, Sfind 2.0,
which implements FDR. We compare Sfind 2.0 with two other source detection and
measurement tasks, Imsad and SExtractor, and comment on several issues arising
from the nature of the correlation between nearby pixels and the necessary
assumption of the null hypothesis. The strong suggestion is made that
implementing FDR as a threshold defining method in other existing
source-detection tasks is easy and worthwhile. We show that the constraint on
the fraction of false detections as specified by FDR holds true even for highly
correlated and realistic images. For the detection of true sources, which are
complex combinations of source-pixels, this constraint appears to be somewhat
less strict. It is still reliable enough, however, for a priori estimates of
the fraction of false source detections to be robust and realistic.Comment: 17 pages, 7 figures, accepted for publication by A
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