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Hot gas has been found to be commonly associated with both individual early-type galaxies and with the poor and rich clusters in which they lie. Although this presentation will concentrate on the hot gas in rich clusters, we briefly describe the characteristics of individual galaxies and groups, as well as clusters since their evolution and present epoch properties are interrelated. Recent reviews of X-ray properties of clusters of galaxies include Forman and Jones (1982) and Sarazin (1986).

Optically bright early-type CE and S0) galaxies, outside the cores of rich clusters, have X-ray luminosities from 1040-1042 ergs sec-1 (in the 0.5-4.5 keV band) which arise from thermal radiation from hot gas at temperatures around 107 K. The gas masses of the individual galaxies lie in the range ~ 109 -1010 Msun (see Forman et al. 1979; Nulsen, Stewart, and Fabian 1984; Forman, Jones, and Tucker 1985; and Canizares, Fabbiano, and Trinchieri 1987 for details). An example of the X-ray observations of two early-type galaxies is shown in Figure 1. The presence of relatively large amounts of hot gas in these galaxies has demonstrated the existence of massive dark halos to gravitationally confine the hot gas (Forman, Jones, and Tucker 1985; Fabian et al. 1986; Mathews and Loewenstein 1986; Sarazin and White 1988). Estimates of the mass-to-light ratio for some galaxies exceed 100 (in solar units).

Figure 1

Figure 1. X-ray contours are shown superposed on the optical photographs of N499 and N507. The X-ray emission around these two galaxies is typical of optically bright E and S0 galaxies. The large gas masses imply that the gas must be in quasi-hydrostatic equilibrium since if the gas were being expelled in a wind the sweeping time is so short that the mass replenishment rates needed to maintain the present X-ray luminosity would be 10-100 times the rates predicted.

The contribution to the X-ray luminosity of early-type galaxies from discrete sources, like those seen in our own galaxy (e.g., neutron star binaries, supernova remnants), is uncertain. Estimates have been derived using observations of Cen A and the bulge of M31 (Forman, Jones, and Tucker 1985; Trinchieri and Fabbiano 1985). In optically faint early-type galaxies (below MB = -19), discrete sources may be the dominant source of X-ray emission since these galaxies may have sufficiently small masses that they cannot gravitationally bind the hot gas. However, no discrete sources have been resolved in any early-type galaxy.

Compact (dense) groups of galaxies (notably those selected by Morgan, Kayser, and White 1975 and Albert, White, and Morgan 1977), have predominantly early-type galaxy populations and are bright in X-rays (Schwartz, Schwarz, and Tucker 1980; Kriss, Cioffi, and Canizares 1983). These groups have gas masses of ~ 1012 - 1013 Msun (within ~ 0.5 Mpc) and gas temperatures up to a few 107 K. Luminosities range up to 1044 ergs sec-1, well into the region populated by rich clusters. An example of the X-ray emission from a compact group is shown in Figure 2. In general, and as the image illustrates for MKW4, the X-ray emission from compact groups is azimuthally symmetric and is centered on the bright D galaxy which dominates the group.

Figure 2

Figure 2. X-ray contours are shown superposed on the optical photograph of the MKW4 group of galaxies (North is up, East to the left). The group has a redshift of 0.0196 and has an X-ray luminosity of 2 x 1044 ergs sec-1. An unresolved source (unrelated to the group) lies to the northwest of the extended emission.

Rich (Abell-like) clusters have X-ray luminosities ranging from as low as those of individual bright galaxies up to 1000 times higher: 1042 - 1045 ergs sec-1 (Jones et al. 1979; Abramopoulos and Ku 1983; Jones and Forman 1984). Gas temperatures range from a few 107 to 108 K (Mushotzky et al. 1978) and gas masses can exceed 1014 Msun within the central few Mpc. The gas densities in the cores of rich clusters lie in the range 10-2 - 10-3 cm-3 and the inferred cooling times of the gas can be as small as 109 years (Fabian, Nulsen and Canizares 1982).

The optical and X-ray images of A1367 (see Figure 3), a richness 2 cluster, emphasize the differences between the two wavelengths. Optically, A1367 appears as an irregularly shaped cluster whose center is difficult to define. There is a noticeable concentration of galaxies to the northwest and a bright galaxy, NGC3862, which is radio and X-ray luminous, is located to the southeast. In X-rays, the cluster structure is clearly seen as an elongated structure with a broad distribution (large core radius).

Figure 3

Figure 3. X-ray contours for A1367 are shown superposed on the optical photograph (see Bechtold et al. 1983 for a detailed discussion of the X-ray properties of A1367). The cluster has an X-ray luminosity of 4 x 1043 ergs sec-1 within a 0.5 Mpc radius (assuming a distance corresponding to z = 0.0213 for H0 = 50 km sec-1 Mpc-1. The galaxy N3862, in the southeast, is bright in X-rays (Elvis et al. 1981). The source in the northwest (near the bright galaxy N3842) is resolved into two sources by the HRI and each is associated with a quasar (Bechtold et al. 1983 and Arp 1984).

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