4.7. Poor clusters
X-ray emission has also been detected associated with poor clusters of galaxies (Schwartz et al., 1980a, b; Kriss et al., 1980, 1981, 1983). Of special interest are the poor clusters that contain D or cD galaxies, lists of which have been given by Morgan et al. (1975) and Albert et al., (1977); clusters from these two lists are identified as MKW and AWM clusters, respectively. The optical properties of these cD galaxies were discussed in Section 2.10.1, where it was concluded that the cDs in poor clusters were similar to cDs in rich clusters, except that they lacked the very extended, low surface brightness envelopes seen in rich cluster cDs (Thuan and Romanishin, 1981; Oemler, 1976; van den Bergh, 1977a). This suggests that the main bodies of cDs are formed by a process, such as mergers, which is not strongly dependent on richness, while the halos are formed by a process, such as tidal interactions, which does depend strongly on richness.
In general, the optical properties of the poor clusters appear to be simple extensions of the properties of rich clusters to lower richness (Stauffer and Spinrad, 1978; Thomas and Batchelor, 1978; Schild and Davis, 1979; Bahcall, 1980; Malumuth and Kriss, 1986). The radio properties of the poor cluster cDs are also very similar to those of rich clusters (White and Burns, 1980; Burns et al., 1980, 1981b).
The poor clusters containing cD galaxies are generally observed to be X-ray sources with luminosities of 1042-44 erg/s (Schwartz et al., 1980a, b; Kriss et al., 1980, 1981, 1983; Malumuth and Kriss, 1986). Kriss et al. (1983) surveyed 16 MKW and AWM clusters, and detected X-ray emission from 12. In the brighter clusters, the X-ray emission was found to be smoothly distributed, relatively symmetrical, and fairly extended (out to radii of 1 Mpc). Kriss et al. found that the X-ray temperatures in these poor clusters were fairly low Tg 1 - 5 keV, in keeping with their low velocity dispersions (see equation 4.10). The X-ray emission is strongly peaked on the position of the cD galaxy (Canizares et al., 1983; Malumuth and Kriss, 1986; see the image of AWM4 in Figure 33). This suggests that the cDs in poor clusters are located at the bottoms of cluster potential wells, as was shown to be the case for rich cluster cDs (Section 2.10.1). In many ways, the bright poor cluster X-ray sources resemble the regular XD clusters discussed above. The X-ray emission in a number of cases is elongated in the same direction as the long axis of the cD galaxy (Kriss et al., 1983; a similar effect is seen in rich clusters, as discussed in Section 2.10.1 and 4.5).
Figure 33. The X-ray emission from the AWM4 poor cluster of galaxies, from Kriss et al. (1983) with the IPC on the Einstein satellite. Contours of constant X-ray surface brightness are shown superimposed on the optical image of the cluster. The emission is centered on the cD galaxy.
From the distribution of the gas in the cluster, the cluster gravitational potential and mass can be derived (Kriss et al., 1983; Malumuth and Kriss, 1986). When these are compared to the optical luminosity of the clusters, a considerable missing mass problem is found. The mass-to-light ratios in the inner parts of these clusters are found to be roughly M/LV 100h50 M / L. The X-ray emitting gas makes up roughly 15% of the total cluster mass in these inner regions. Because the cD galaxy contributes a large fraction of the luminosity in these poor clusters, this dark matter can also be thought of as a massive halo around the central cD, as in the M87/Virgo cluster.
The X-ray emission is strongly peaked on the position of the cD in these poor clusters (Canizares et al., 1983; Kriss et al., 1983; Malumuth and Kriss, 1986), and the cooling times in the central portions of the gas are generally estimated to be less than the probable age of the cluster (the Hubble time). This suggests that the X-ray emitting gas forms a cooling accretion flow onto the cD (Canizares et al., 1983), as has been observed in many rich clusters. The accretion rates in MKW4, MKW3s, AWM4, and AWM7 are estimated to be in the range of 5-100 M / yr (Canizares et al., 1983; Malumuth and Kriss, 1986). It is possible that star formation from the accretion flow may provide a portion of the optical luminosity of the cD (Fabian et al., 1982b; Sarazin and O'Connell, 1983). Very little cold gas (neutral hydrogen) is detected in these cDs (Burns et al., 1981a; Valentijn and Giovanelli, 1982).
Poor clusters containing head-tail radio sources have also been detected in X-rays (Burns and Owen, 1979; Holman and McKee, 1981). Head-tail radio sources are believed to be produced by galaxy motions through intracluster gas (Sec 4.3).