Cluster Evolution

The so far the best studies on cluster evolution are based on the nearby and distant cluster samples compiled from serendipitous cluster discoveries in the FoV of Einstein and ROSAT archival observations [21,74,105,125,126,150]. For the older Einstein based sample (EMSS), detailed data were compiled in extensive follow-up observations including temperature functions for various redshifts which provide a very sensitive test of the evolution of the cluster population [74]. These data show an evolutionary trend which can approximately be explained in the frame of the concordance cosmological and current structure formation models. The ROSAT archive based distant cluster surveys reach out to larger redshifts. The largest number of high redshift clusters are contained in the ROSAT Distant Cluster Survey (RDCS) by Rosati et al. [125], while the largest number and best statistics comes from the 160deg2 survey lead by Vikhlinin [150]. The evolutionary effect in the luminosity function found in the RDCS is shown in Fig. 23.27 (top). There is a significant trend of a decreasing cluster abundance at the high luminosity end of the X-ray luminosity function, that is a negative evolution with redshift. This negative evolution taken together with the expected brightening of galaxy clusters of a given mass with redshift is a clear indication of the decreasing abundance of massive clusters with look-back time and lends therefore strong support to the hierarchical formation model of galaxy clusters from smaller to increasingly larger units. A similar conclusion with high significance is also obtained from the analysis of the 160 deg2 survey [105].

The constraints on cosmological models from these results on cluster evolution are shown Fig.23.27 (bottom) [21]. The figure shows results for various

Fig. 23.27 Top: Evolution of the cluster X-ray luminosity function observed in the RDCS survey by Rosati et al. [126]. The abundance of the most X-ray luminous clusters is clearly lower at high redshift. Bottom: Constraints on the cosmological density parameter, Q.m, and the amplitude of the matter density fluctuations on a scale of 8h-1 Mpc, 08, from the RDCS Survey for different assumptions on the mass-X-ray luminosity relation. The parameter ¡5 refers to the amplitude of the mass-temperature relation, while the other two parameters modify the temperature-luminosity relation, LX « Tyf (1 + z)A. For details see Borgani et al. [21]

Fig. 23.27 Top: Evolution of the cluster X-ray luminosity function observed in the RDCS survey by Rosati et al. [126]. The abundance of the most X-ray luminous clusters is clearly lower at high redshift. Bottom: Constraints on the cosmological density parameter, Q.m, and the amplitude of the matter density fluctuations on a scale of 8h-1 Mpc, 08, from the RDCS Survey for different assumptions on the mass-X-ray luminosity relation. The parameter ¡5 refers to the amplitude of the mass-temperature relation, while the other two parameters modify the temperature-luminosity relation, LX « Tyf (1 + z)A. For details see Borgani et al. [21]

assumptions on how the cluster luminosity evolves for given cluster mass and temperature. They clearly favor a low density universe for the whole range of evolutionary parameters considered.

The high sensitivity of XMM-Newton is now providing a deeper look into the X-ray sky and widens the horizon to the study of even more distant galaxy clusters. So far the most distant X-ray cluster has recently been detected at a redshift of 1.39 [106]. Most surprisingly, optical and nearlR imaging shows a cluster with a

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Fig. 23.28 Left: The most distant X-ray cluster found to date at a redshift of z = 1-39. The object was detected in a search for distant X-ray clusters in the fields of archival XMM-Newton observations [106]. The figure shows an optical/nearlR composite image with X-ray contours superposed. Most remarkably, even at this large look-back time of 9 Gyrs, the major cluster galaxies show very red colors, the signature of very old stellar populations

very old red population of galaxies and also the X-ray appearance of the cluster tells us that this system is already well evolved (Fig. 23.28). A large XMM-Newton survey program, the XMM-LSS, is also devoted to a distant cluster search [115].

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