The X-ray View of Galaxy Clusters

Smithsonian Astrophysical Observatory Harvard-Smithsonian Center for Astrophysics Cambridge, MA, USA wrf cfa.harvard.edu 

Clusters of galaxies are the most massive collapsed systems in the Universe. A typical luminous cluster (e.g., Coma cluster) is filled with a hot, 100 million degree, low density (10-3 cm-3) gas. In addition to the optically luminous galaxies and diffuse X-ray gas, clusters are dominated by dark matter. The X-ray gas, relaxing on the relatively short sound crossing time, tmpc = 6.6 x 108(T/108) yrs(wherer = D/cs and cj. = fP/ p) is an effective tracer of this unseen dark matter. 

Cluster studies provide a unique window on the Universe. Although clusters are the most massive collapsed systems in the Universe, with dynamical timescales of order 109 yrs in their cores, they are relatively young and remember the conditions from which they formed. Also, clusters form from rare overdensities. Therefore, cluster properties and numbers are sensitive to cosmological parameters. 

Clusters form from large volumes. For a Universe with a mean mass density of 3 x 10 3° gm cm-3 (30% of closure density pc = 3 //HirG, with Ho=70 km s-1), a rich cluster with a mass of 1015M forms from a sphere with a radius of 20 Mpc. Since the dominant process in the formation of the cluster whose mass consists of cold dark matter and baryons is gravitation and the formation of collapsed objects by gravity alone should not affect the ratio of the mass components, it is believed that the mass components of today s clusters are representative of the Universe (e.g., White et al. 1993). 

The mass fluctuation spectrum gives rise to collapsed objects from galaxies to groups to rich clusters. The systems dominated by old stars - early type galaxies, elliptical-dominated groups, and rich clusters - all have gaseous halos 

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