The X-ray spectra of clusters of galaxies

The identification of extended sources of X-ray emission associated with clusters of galaxies has been one of the major achievements of X-ray astronomy. In this paper the present situation regarding the identification of X-ray clusters is reviewed and the relationship between X-ray and optical cluster properties is briefly discussed. Measurements of X-ray extent have been made for a small number of clusters and the status of these observations is described. The nature of the X-ray emission process and the observations which have led to our present understanding of cluster X-ray production are examined in some detail. Observations at other wavelengths that are relevant to this problem are also discussed. The available evidence suggests that the X-ray emission is due to bremsstrahlung from high-temperature (T > 2 x 107 K) gas that is present in the space between cluster galaxies. Current models of the origin of this gas and of its likely heating mechanism are presented. (orig.) 891 HK/orig. 892 MB