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Santos, Joana (2008): The baryon content of distant X-ray Galaxy Clusters. Dissertation, LMU München: Fakultät für Physik



In the framework of the current cosmological paradigm, cosmic evolution is mostly driven by gravity through the hierarchical growth of cold dark matter structures. However, the evolution of the directly observed luminous component involves complex non-gravitational processes such as cooling, star formation and feedback mechanisms involving the conventional matter well known to us, termed shortly as baryons. Clusters of galaxies are the largest virialized systems in the Universe, hence are ideal laboratories to study the evolution of baryons. The baryon content of clusters accounts for roughly 15% of their total mass, encompassing a “cold phase” in the form of luminous galaxy masses, and a “hot phase” corresponding to the X-ray emitting intracluster medium (ICM). The thermodynamics of baryons is affected by non-trivial phenomena and the interplay of the intricate processes between these two phases remains, to a large extent, unclear. In this thesis I investigate the properties of both the ICM and the underlying galaxy populations in X-ray selected distant clusters, with the aim of constraining physical processes governing the evolution of clusters and their galaxies. The inner regions of local clusters often exhibit radiative cooling, termed cool cores (CC). I have made an important step in investigating the abundance of cool cores in the distant cluster population, by devising efficient methods to characterize local CCs, that were applied to the highest redshift cluster sample currently available (0.7 < z < 1.4) from the Chandra archive. The fraction of CCs seems to decrease with redshift, since I find that the majority of the distant clusters are in an intermediate state of cooling. High-z (z∼ >1) clusters are hard to find. The XMM-Newton Distant Cluster Project (XDCP) is a survey aimed to construct a complete sample of z∼1 clusters from the XMM-Newton archive. Within this scope a large effort has been done to confirm potential distant cluster candidates by exploring a new optical & near-infrared imaging technique to identify overdensities of galaxies. Twenty-two cluster candidates were imaged during two runs at the ESO/La Silla Observatory, of which around half are potential distant clusters, based on their I-H images. The applied photometric technique has thus proven to be reliable in identifying z≥0.8 overdensities of galaxies. The formation and evolution of massive early-type galaxies (ETGs) is still an open question, since the observational data cannot be easily reconciled with the preferred, hierarchical galaxy formation scenario. Using high resolution Hubble Space Telescope/ACS imaging and VLT/FORS2 spectra, I studied the galaxy population of XMM1229 at z=0.975, discovered in the XDCP. The results show a red-sequence populated by galaxies with stellar masses in the range 5e10 - 2e11 solar masses and old (3-4 Gyr) underlying stellar population formed at zf ∼ 4. The color-magnitude relation at this high redshift is found to be already very tight (with a 0.04 spread similar to the local Coma cluster). This confirms that ETGs in clusters assembled early on and in short timescales, and their star formation processes have already completed the essential part of their chemical enrichment, as elucidated by the high metal abundance (Z ∼ 0.3 solar) of the ICM, measured with XMM spectra.