Annu. Rev. Astron. Astrophys. 1989. 27: 87-138 Copyright © 1989 by Annual Reviews. All rights reserved

1. INTRODUCTION

The study of the X-ray properties of normal galaxies as a class was made possible by the launch of the Einstein Observatory in November 1978 (Giacconi et al. 1979). Before then, with the exclusion of the bright X-ray sources associated with Seyfert nuclei (Elvis et al. 1978, Tananbaum et al. 1978), only four galaxies had been detected in X rays: the Milky Way, M31, and the Magellanic Clouds (see Helfand 1984a, and references therein). The Einstein X-ray observations of well over 100 galaxies have been reported in the literature to date, and data on a similar number can still be found in the Einstein data bank. Some galaxies were detected with enough detail to allow a study of their X-ray morphology, spectra, and individual sources and to make comparisons with optical, infrared, and radio data. For all the galaxies, values of the X-ray flux, or even upper limits to this flux in the case of nondetections, can be used to explore average sample properties. These observations have shown that normal galaxies of all morphological types are spatially extended sources of X-ray emission with luminosities in the range of ~ 10³⁸ erg s^-1 to 10⁴² erg s^-1. Although this is only a small fraction of the total energy output of a normal galaxy, X-ray observations are uniquely suited to study phenomena that are otherwise elusive. These include the end products of stellar evolution (supernova remnants and compact remnants) and a hot phase of the interstellar medium, discovered in bright early-type galaxies. This review gives an up to date and hopefully complete account of these Einstein results and of the published results from the European EXOSAT and Japanese Ginga satellites. Spiral and elliptical galaxies are reviewed separately, since their X-ray properties differ and suggest different origins for their X-ray emission.