Annu. Rev. Astron. Astrophys. 1982. 20: 547-85 Copyright © 1982 by Annual Reviews. All rights reserved

1. INTRODUCTION

The X-ray imaging capability of the Einstein Observatory has provided new observational material for many branches of astronomy. Observations of galaxy clusters allow one to address problems relating to the member galaxies, including their interactions with the intracluster gas, their masses and mass distributions, their origins and evolution, and the galactic accretion of cooling intracluster gas. One can also study global cluster properties, such as the distribution of the virial mass, the composition of the intracluster medium, the dynamical state of clusters, and their evolution with redshift. In addition, the observations can be used to perform cosmological measurements.

The fundamental properties of clusters of galaxies have been reviewed by Abell (1975) and Bahcall (1977a). Gursky & Schwartz (1977) discussed the early X-ray observations, including the first images made with rocket-borne instrumentation and the initial suggestions for emission mechanisms. In this volume, Holt & McCray (1982) review X-ray spectroscopic observations. Here we present Einstein X-ray imaging observations to illustrate the variety of phenomena that can be addressed through the analysis of the X-ray images. We first briefly describe the Einstein focal-plane instruments and then discuss cluster dynamical timescales. In Section 2, we review general cluster properties and give a simple description of the intracluster gas. Sections 3 and 4 contain detailed discussions of individual clusters. In Section 5, we review cluster classification and dynamical evolution. The use of X-ray images to determine the cluster mass distribution is then discussed in Section 6, and X-ray observations of distant clusters are reviewed in Section 7.