Clusters of Galaxies

Abstract

Associations of galaxies range from pairs and small groups, through the giant clusters containing over a thousand galaxies, to the vast structures on scales much greater than clusters such as the vast ‘walls’ seen in Figs. 2.7 and 2.8. Clustering occurs on all scales, as is demonstrated by the two-point correlation function for galaxies (Figs. 2.5 and 2.6). Few galaxies can be considered truly isolated. Rich clusters of galaxies are of particular interest because they are the largest gravitationally bound systems we know of in the Universe. They possess correspondingly deep gravitational potential wells which can be observed through the bremsstrahlung Xray emission of hot gas which forms an atmosphere within the cluster. The hot gas can also be detected through the decrements which it causes in the Cosmic Microwave Background Radiation as a result of the Sunyaev–Zeldovich effect.