Ray Cluster Surveys

The systematic identification of galaxy clusters in X-rays across the sky has received an enormous boost by the ROSAT All-Sky Survey (RASS) [149,154], the first X-ray all-sky survey conducted with an imaging X-ray telescope and the comprehensive archive of ROSAT pointed observations. Before that event a few hundred galaxy clusters were detected in X-rays and studied mainly with the Einstein and HEAO-1 satellite observatories. The RASS Atlas contains, as estimated, 8 000-10 000 galaxy cluster X-ray sources, of which about 2 000 have been identified by now. Recent reviews by Schuecker [133] and Edge [46] provide a comprehensive account of pre-ROSAT and ROSAT-based cluster surveys, e.g. [11,13,16,43,44,68].

The sample of the X-ray brightest ^100 galaxy clusters (outside the zone-of-avoidance around the plane of the Milky Way) has presumably been detected completely and compiled by Reiprich and Bohringer in the HIFLUGCS sample [123] from which the first empirical cluster mass function has been determined. Figure 23.22 (right) shows the cumulative mass density of the clusters as a function of the lower mass limit in units of the critical density of the Universe. Only about 2% of the critical density, that is, about 6% of the matter density of the Universe (assuming a concordance cosmological model with Qm = 0.3) is found in galaxy clusters and groups of galaxies with a mass above about 2.1 x 1013^701 M0 [123] . Nevertheless, this small mass fraction provides tight constraints on the large-scale structure parameters of the matter distribution in the Universe.

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Fig. 23.22 Left: Mass-X-ray luminosity relation determined from the 106 brightest galaxy clusters found in the ROSAT All-Sky Survey by Reiprich and Bohringer [123]. Right: Mass density of galaxy clusters in the Universe as a function of the lower mass cut in units of the critical density of the Universe [123]

Fig. 23.23 Sky distribution of the brightest galaxy clusters found in the ROSAT All-Sky Survey investigated in the NORAS [11] and REFLEX [16] cluster redshift surveys

REfLEX

Fig. 23.23 Sky distribution of the brightest galaxy clusters found in the ROSAT All-Sky Survey investigated in the NORAS [11] and REFLEX [16] cluster redshift surveys

X-ray surveys of clusters provide several advantages for the construction of cluster samples to be used for cosmological studies. First of all the X-ray luminosity is tightly correlated to the cluster mass as shown in Fig. 23.22 (left) [123], and the presence of X-ray emission is a signature of a truly bound structure, and due to the centrally peaked surface brightness profile projection effects on the sky are minimized. For the comparison of the observed cluster abundances and spatial distributions with theoretical model predictions the masses of the clusters involved have to be known, at least approximately. This is therefore currently best provided by X-ray observations, as demonstrated in Fig. 23.22.

For the present illustration of the use of galaxy clusters as cosmological probes we use the REFLEX cluster survey, the currently largest, most homogeneous, and most complete X-ray cluster survey [13,16]. This survey covers the southern sky up to declination 8 = +2.5°, avoiding the band of the Milky Way (\bu\ < 20°) and the regions of the Magellanic clouds with a total survey area of 13 924 deg2 or 4.24 sr (Fig. 23.23). The X-ray detection of the clusters is based on the second processing of the RASS [154] with a reanalysis of the X-ray properties of the sources by means of the growth curve analysis method [11]. The current sample has a flux-limit of Fx > 3 x 10-12 erg s^1 cm~2 and comprises 447 objects. An extension of the sample termed REFLEX II to a lower flux limit is almost completed. A comprehensive optical follow-up program in the frame of an ESO key program was used to definitely identify the so far unknown X-ray cluster candidates and to measure the missing cluster redshifts. The final cluster catalogue is estimated to be better than 90% complete with a contamination from X-ray luminous AGN less than 9%. A series of tests demonstrate the high quality of the sample. Figure 23.25 (left) shows for example the Gaussianity of the cluster distribution of a study similar to counts

Fig. 23.24 Left: Slice through the three-dimensional cluster distribution in NORAS and REFLEX for a flux limit of 3 x 10~12 erg cm~2. The slice is taken perpendicular to the galactic plane (along ln = 90-270 deg). Some famous cluster concentrations in the local Universe are indicated (G.A. marks the Great Attractor region). Right: Distribution of the galaxy clusters in the extended REFLEX and NORAS surveys out to redshifts of about z = 0.2 containing about 1400 galaxy clusters. The clumpy structure of the cluster distribution is easily visible

Fig. 23.24 Left: Slice through the three-dimensional cluster distribution in NORAS and REFLEX for a flux limit of 3 x 10~12 erg cm~2. The slice is taken perpendicular to the galactic plane (along ln = 90-270 deg). Some famous cluster concentrations in the local Universe are indicated (G.A. marks the Great Attractor region). Right: Distribution of the galaxy clusters in the extended REFLEX and NORAS surveys out to redshifts of about z = 0.2 containing about 1400 galaxy clusters. The clumpy structure of the cluster distribution is easily visible in cells conducted in connection with the KL-structure analysis described further (see [131]).

The three-dimensional cluster distribution is sketched in Fig. 23.24, which shows a slice through the space distribution of the REFLEX and NORAS3 clusters, which includes the region of the Milky Way. The other panel in Fig. 23.24 shows a view on a three-dimensional model of the cluster distribution in the extended REFLEX and NORAS surveys. Both pictures clearly show the clumpy structure of the cluster distribution, which results in a higher than random probability to find a cluster near another cluster than at any random point in space as reflected by the two-point correlation function [38].

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