Search

Galaxy clusters are ideal probes of the large-scale structure of the Universe and for the tests of cosmological models. Based on the REFLEX redshift survey of X-ray selected clusters of galaxies we determine statistical properties of the galaxy cluster population, their spatial correlation, and the density fluctuation power spectrum of the cosmic matter distribution on large scales up to about 1 Gpc. Comparing these results with predictions of cosmological models we obtain tight constrains for the matter density parameter of the Universe, consistent with the combined results from observations of the microwave background anisotropies and distant type Ia supernovae. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The construction of X-ray cluster surveys is described. As an example the selection processes applied to the REFLEX survey and first results of an analysis of the spatial distributions of the clusters are given in more detail. [ABSTRACT FROM AUTHOR]

The halo of M 87 is an interesting study object for X-ray and radio astronomy. X-ray observations allow to study the hot gaseous environment of M 87 and provide a means to determine the galaxy halo mass. The X-ray halo is actually an indistinguishable part of the intracluster medium of the Virgo cluster, which features a moderate cooling flow focused onto M 87. There is a clear interaction effect between the relativistic plasma in the radio lobes and the thermal intracluster medium, featuring cooling gas in the regions of the lobes. [ABSTRACT FROM AUTHOR]

This paper provides a brief review on ionization and radiation models of an optically thin, hot plasma and their application to the interpretation of observations of the Local Bubble. The basic structure and atomic physics ingredients of the models are outlined. Some general characteristic results of ionization equilibrium and non-equilibrium calculations are shown. Recent progress in plasma model codes is reviewed. The interpretation of the observations of absorption lines and emission spectra from the Local Bubble is discussed in the framework of ionization equilibrium and non-equilibrium ionization models with particular reference to the fast adiabatic cooling model of Breitschwerdt and Schmutzler. [ABSTRACT FROM AUTHOR]

The cosmologically most important cluster parameter is its gravitational mass. In order to constrain cosmological models of structure formation by comparing simulations or analytic models with, e.g., observational mass functions of statistical cluster samples, cluster masses are needed. Precise cluster mass determinations for large cluster surveys, e.g. [3], however, require extensive observations and are currently not feasible. An alternative approach is to use established relations between more easily observed quantities and cluster mass. For optical cluster samples the observed number of cluster galaxies has sometimes been used to relate to cluster mass. For X-ray cluster samples a three component approach is commonly used. First, measured X-ray luminosities are converted to X-ray temperatures using an observational relation. Then, a slope for the X-ray temperature - gravitational mass relation is assumed on theoretical grounds. And finally, the X-ray temperatures are converted to masses by normalizing this relation with hydrodynamical simulations. [ABSTRACT FROM AUTHOR]

Solid observational evidences indicate a strong dependence of the galaxy formation and evolution on the environment. In order to study in particular the interaction between the intracluster medium and the evolution of cluster galaxies, we have created a large database of clusters of galaxies based on the largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS), and the Sloan Digital Sky Survey (SDSS). We analyzed the correlation between the total optical and the X-ray cluster luminosity. The resulting correlation of LX and Lop shows a logarithmic slope of 0.6, a value close to the self-similar correlation. We analysed also the cluster mass to light ratio, by finding a significant dependence of M/L on the cluster mass with a logarithmic slope ranging from 0.27 in the i and r bands to 0.22 in the z band. [ABSTRACT FROM AUTHOR]

A new X-ray selected/X-ray flux-limited galaxy cluster sample of the 63 X-ray brightest clusters in the sky has been compiled from recent X-ray cluster catalogs based on the ROSAT All-Sky Survey. Cluster masses have been determined homogeneously using mainly ROSAT pointed observations and mostly ASCA gas temperatures, assuming the intracluster gas to be in hydrostatic equilibrium. As the sample is statistically complete, a cluster mass function has been constructed. Integration of the mass function shows that the total gravitating mass contained within the virial radius of clusters with mass larger than $3.5 \times 10^{13} h^{-1}_{50} M_\odot$ ($\sim$ Hickson group mass) is about two percent of the total mass in a critical density universe, i.e. $\Omega_{cluster} \approx 0.02$, and about six percent for a normalized matter density $\Omega_m = 0.3$. This implies that by far most of the total mass in the universe resides outside virialized cluster regions. The mass fraction given by Fukugita et al., ApJ, 503, 518 (1998), based on the mass function determined by Bahcall & Cen, ApJ, 407, L49 (1993), is four times higher than the value found here at the corresponding minimum mass. The errors are currently being investigated. [ABSTRACT FROM AUTHOR]