2,594 research outputs found
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
The Mean and Scatter of the Velocity Dispersion-Optical Richness Relation for maxBCG Galaxy Clusters
The distribution of galaxies in position and velocity around the centers of
galaxy clusters encodes important information about cluster mass and structure.
Using the maxBCG galaxy cluster catalog identified from imaging data obtained
in the Sloan Digital Sky Survey, we study the BCG-galaxy velocity correlation
function. By modeling its non-Gaussianity, we measure the mean and scatter in
velocity dispersion at fixed richness. The mean velocity dispersion increases
from 202+/-10 km/s for small groups to more than 854+/-102 km/s for large
clusters. We show the scatter to be at most 40.5+/-3.5%, declining to
14.9+/-9.4% in the richest bins. We test our methods in the C4 cluster catalog,
a spectroscopic cluster catalog produced from the Sloan Digital Sky Survey DR2
spectroscopic sample, and in mock galaxy catalogs constructed from N-body
simulations. Our methods are robust, measuring the scatter to well within
one-sigma of the true value, and the mean to within 10%, in the mock catalogs.
By convolving the scatter in velocity dispersion at fixed richness with the
observed richness space density function, we measure the velocity dispersion
function of the maxBCG galaxy clusters. Although velocity dispersion and
richness do not form a true mass-observable relation, the relationship between
velocity dispersion and mass is theoretically well characterized and has low
scatter. Thus our results provide a key link between theory and observations up
to the velocity bias between dark matter and galaxies.Comment: 25 pages, 15 figures, 2 tables, published in Ap
The Bright SHARC Survey: The Cluster Catalog
We present the Bright SHARC (Serendipitous High-Redshift Archival ROSAT
Cluster) Survey, which is an objective search for serendipitously detected
extended X-ray sources in 460 deep ROSAT PSPC pointings. The Bright SHARC
Survey covers an area of 178.6 sq.deg and has yielded 374 extended sources. We
discuss the X-ray data reduction, the candidate selection and present results
from our on-going optical follow-up campaign. The optical follow-up
concentrates on the brightest 94 of the 374 extended sources and is now 97%
complete. We have identified thirty-seven clusters of galaxies, for which we
present redshifts and luminosities. The clusters span a redshift range of
0.0696<z<0.83 and a luminosity range of 0.065<Lx<8.3e44 erg/s [0.5-2.0 keV]
(assuming Ho = 50 km/s/Mpc and qo=0.5). Twelve of the clusters have redshifts
greater than z=0.3, eight of which are at luminosities brighter than Lx=3e44
erg/s. Seventeen of the 37 optically confirmed Bright SHARC clusters have not
been listed in any previously published catalog. We also report the discovery
of three candidate ``fossil groups'' of the kind proposed by Ponman et al.
(1994).Comment: Minor revisions: References updated and typos corrected. Shortened by
use of emulateapj.st
SDSS-RASS: Next Generation of Cluster-Finding Algorithms
We outline here the next generation of cluster-finding algorithms. We show
how advances in Computer Science and Statistics have helped develop robust,
fast algorithms for finding clusters of galaxies in large multi-dimensional
astronomical databases like the Sloan Digital Sky Survey (SDSS). Specifically,
this paper presents four new advances: (1) A new semi-parametric algorithm -
nicknamed ``C4'' - for jointly finding clusters of galaxies in the SDSS and
ROSAT All-Sky Survey databases; (2) The introduction of the False Discovery
Rate into Astronomy; (3) The role of kernel shape in optimizing cluster
detection; (4) A new determination of the X-ray Cluster Luminosity Function
which has bearing on the existence of a ``deficit'' of high redshift, high
luminosity clusters. This research is part of our ``Computational
AstroStatistics'' collaboration (see Nichol et al. 2000) and the algorithms and
techniques discussed herein will form part of the ``Virtual Observatory''
analysis toolkit.Comment: To appear in Proceedings of MPA/MPE/ESO Conference "Mining the Sky",
July 31 - August 4, 2000, Garching, German
The XMM-Newton Project
The abundance of high-redshift galaxy clusters depends sensitively on the
matter density \OmM and, to a lesser extent, on the cosmological constant
. Measurements of this abundance therefore constrain these fundamental
cosmological parameters, and in a manner independent and complementary to other
methods, such as observations of the cosmic microwave background and distance
measurements. Cluster abundance is best measured by the X-ray temperature
function, as opposed to luminosity, because temperature and mass are tightly
correlated, as demonstrated by numerical simulations. Taking advantage of the
sensitivity of XMM-Newton, our Guaranteed Time program aims at measuring the
temperature of the highest redshift (z>0.4) SHARC clusters, with the ultimate
goal of constraining both \OmM and .Comment: To appear in the Proceedings of the XXI Moriond Conference: Galaxy
Clusters and the High Redshift Universe Observed in X-rays, edited by D.
Neumann, F. Durret, & J. Tran Thanh Va
Lensing-Induced Structure of Submillimeter Sources: Implications for the Microwave Background
We consider the effect of lensing by galaxy clusters on the angular
distribution of submillimeter wavelength objects. While lensing does not change
the total flux and number counts of submillimeter sources, it can affect the
number counts and fluxes of flux-limited samples. Therefore imposing a flux cut
on point sources not only reduces the overall Poisson noise, but imprints the
correlations between lensing clusters on the unresolved flux distribution.
Using a simple model, we quantify the lensing anisotropy induced in
flux-limited samples and compare this to Poisson noise. We find that while the
level of induced anisotropies on the scale of the cluster angular correlation
length is comparable to Poisson noise for a slowly evolving cluster model, it
is negligible for more realistic models of cluster evolution. Thus the removal
of point sources is not expected to induce measurable structure in the
microwave or far-infrared backgrounds.Comment: 22 pages, 9 figures, accepted to Astrophysical Journa
Shape-Selective Supramolecular Capsules for Actinide Precipitation and Separation
Improving actinide separations is key to reducing barriers to medical and industrial actinide isotope production and to addressing the challenges associated with the reprocessing of spent nuclear fuel. Here, we report the first example of a supramolecular anion recognition process that can achieve this goal. We have designed a preorganized triamidoarene receptor that induces quantitative precipitation of the early actinides Th(IV), Np(IV), and Pu(IV) from industrially relevant conditions through the formation of self-assembled hydrogen-bonded capsules. Selectivity over the later An(III) elements is shown through modulation of the nitric acid concentration, and no precipitation of actinyl or transition-metal ions occurs. The Np, Pu, and Am precipitates were characterized structurally by single-crystal X-ray diffraction and reveal shape specificity of the internal hydrogen-bonding array for the encapsulated hexanitratometalates. This work complements ion-exchange resins for 5f-element separations and illustrates the significant potential of supramolecular separation methods that target anionic actinide species
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