|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
X-ray emission has been detected from over one hundred nearby (z 0.08) clusters (McHardy 1978, Jones & Forman 1978, McKee et al. 1980, Helfand et al. 1980, Murray et al. 1982, White & Silk 1980, Nulsen et al. 1982, Forman et al. 1981a, Ulmer et al. 1979, Jones et al. 1979, Jones & Forman, in preparation, Piccinotti et al. 1982). The cluster X-ray luminosities (0.5-3.0 keV) range from 1042 to 1045 erg s-1 within 0.5 Mpc of the cluster centers. (H0 = 50 km s-1 Mpc-1 and q0 = 0.0 are assumed throughout this paper.) From HEAO A-2 observations of Abell clusters with distance class 4 or less, McKee et al. (1980), Hintzen et al. (1980), and Piccinotti et al. (1982) determined the X-ray luminosity function and found the contribution of clusters to the X-ray background to be ~ 4-9% in the 2-10 keV energy band.
There is little doubt that the dominant X-ray emission process from clusters in the energy range below 10 keV is thermal bremsstrahlung. The strongest evidence for a thermal origin is the detection of emission features resulting from transitions of highly ionized iron in the spectra of 18 clusters (Mitchell et al. 1976, Serlemitsos et al. 1977, Mitchell & Culhane 1977, Berthelsdorf & Culhane 1979, Mushotzky et al. 1978, Mushotzky & Smith 1980). All are consistent with the same Fe/H abundance (by number), about one half the solar value (Smith et al. 1979). For 26 clusters, gas temperatures are observed to range from ~ 2 to ~ 10 keV (Mushotzky & Smith 1980). Observations of head-tail radio sources in clusters (e.g. Miley et al. 1972, Harris 1980) support the hot-plasma hypothesis, since the density and temperature of the intracluster gas required to contain the tails agree with values computed for thermal X-ray emission. Indications of microwave decrements (Sunyaev & Zel'dovich 1972) have been reported in Abell 576, Abell 2218, and 0016 + 16 (Birkinshaw et al. 1978, Lake & Partridge 1977, Lake & Partridge 1980, Birkinshaw et al. 1981a, Birkinshaw et al. 1981b). The intensity reduction longward of the blackbody peak and the increased intensity at submillimeter wavelengths were predicted to be caused by Compton scattering of microwave background photons on electrons in a hot intracluster gas. Although uncertainties in the detections linger, the reported decreases in background brightness temperatures are consistent with a cluster electron density of ~ 10-3 cm-3, comparable to densities derived from models of hot, X-ray-emitting, cluster gas.
The discovery of the presence of iron with nearly cosmic abundance in the intracluster gas suggests that gas injection is important in the origin of the gas. The injection may result from an early generation of massive stars (DeYoung 1978), galactic winds (Mathews & Baker 1971), ram-pressure stripping (Gunn & Gott 1972), or evaporation (Cowie & Songaila 1977). Primordial gas also may contribute to the intracluster medium either from inefficient galaxy formation or from accretion into the cluster (Gunn & Gott 1972). Intergalactic gas may be provided by all of these mechanisms.