The structure of clusters of galaxies observed with Einstein

Einstein imaging observations have been used in deriving surface brightness profiles of the X-ray emission from 46 clusters of galaxies. By fitting these profiles to hydrostatic-isothermal models of the intracluster gas, the authors have determined the X-ray luminosity and derived the cluster core radius, the ratio of the scale height of the galaxies to that of the gas, and the central gas density. This analysis shows that clusters of galaxies can be divided into two families. Clusters in the XD family have small core radii (⪉300 kpc) with the X-ray emission centered on a central, stationary, optically dominant galaxy. Clusters classed nXD have larger core radii (≡400 to ≡800 kpc), and generally the emission is not centered on a stationary, bright galaxy. An extensive discussion of the dynamical properties and the gas content of the two families is given and it is concluded that the formation of a central, dominant galaxy occurs in the early stages of cluster collapse.