Annu. Rev. Astron. Astrophys. 1994. 32: 115-52 Copyright © 1994 by Annual Reviews. All rights reserved

X-ray Emission

Most of our understanding of the X-ray properties of galaxies comes from the results of Einstein Observatory measurements in the 0.2-3.5 keV X-ray band. A detailed review of X-rays from normal galaxies is given by Fabbiano (1989) while a briefer summary is given by Sarazin (1992). Catalogs include those by Fabbiano et al. (1992) and Roberts et al. (1991). ROSAT data and reports are appearing as this review is being written. In the near future we can expect important extensions to our knowledge of X-ray sources.

X-rays have been detected from all galaxy types except the low luminosity dE and dS0 classes. ROSAT detections of X-rays in the direction of such dwarfs appear to be background sources (e.g., Gizis et al. 1993). There is a correspondence between optical and X-ray luminosities such that only the nearest Im-type systems, those in the Local Group, have thus far been seen in X-rays. The optical (blue)-X-ray luminosity dependence differs with galaxy type. E's and S0's have, with large dispersion, an X-ray luminosity which varies approximately as the square of their optical luminosity (Canizares et al. 1987; Bregman 1992). In contrast, the late-type galaxies show, again with large dispersion, a linear dependence between these two quantities (Fabbiano et al. 1988).

Here we limit our discussion ton ormal galaxies, which have X-ray luminosities in the range 10³⁸-10⁴² ergs s^-1. The X-ray spectrum differs with galaxy type being harder for spirals. There are two distinctly different origins of the (non-nuclear) radiation: (a) a component which is basically stellar in origin: supernova remnants and X-ray binaries, and (b) diffuse emission from hot 10⁶-10⁷ K, gas.

In spirals, the stellar constituent can have both Population I and II components: for the former, supernova remnants and high mass X-ray binaries and for Population II, the so-called low-mass X-ray binaries. In our Galaxy, low-mass binaries are found primarily in globular clusters and in the central bulge region although some have also been identified in the disk. The million degree gas is the principal source of X-rays in the luminous early-type galaxies. This thermal plasma has its origin in the mass lost by evolving stars, stars which in the bulge component of early-type systems have random motions of typically a few hundred km s^-1. Ejecta from these evolving stars encounter at velocities which correspond to the observed X-ray temperature. The recognition that X-rays are so prominently emitted by ellipticals solves the long recognized dilemma of locating the mass lost in evolution in such stellar-rich systems.

In summary, X-rays are seen from all galaxy types with the properties of radiation dependent on the fraction of the Population types in each galaxy. This variation is exemplified by bright ellipticals with relatively high luminosity soft X-rays and by the bright, late-type spirals with lower luminosity, harder X-radiation. The Sa's with prominent bulges as well as star-forming disks have X-ray characteristics between those of E's and the later spirals.