4.8. High redshift clusters and X-ray cluster evolution
Observations of high redshift X-ray clusters (taken here to mean clusters with z 0.2) offer the possibility of studying the formation and evolution of clusters. At the moment this study is hampered by the small sample of clusters that have been observed and the difficulty in obtaining detailed information (spectra and spatial distributions) for these distant sources. Samples of high redshift X-ray clusters have been given by Henry et al. (1979, 1982), Helfand et al. (1980), Perrenod and Henry (1981), White et al. (1981a, b, 1987), and Henry and Lavery (1984). These samples are based on lists of clusters detected initially through optical or radio emission.
One purpose of this study is to determine whether X-ray clusters have evolved in any detectable way over the cosmological time span represented by the redshift. Henry et al. (1979) compared a sample of six high redshift clusters with more nearby clusters, and concluded that they were similar in many ways. They found marginal evidence that X-ray luminosities increase with time in accordance with the models of Perrenod (1978b) and Sarazin (1979), but the sample was too small to make any strong statements. Henry et al. (1982) and Henry and Lavery (1984) studied the evolution of the X-ray luminosity function at redshifts z < 0.5. No significant evidence for evolution was found.
Perrenod and Henry (1981) estimated gas temperatures for a sample of seven high redshift clusters observed with the Einstein observatory. With one exception (the remarkable cluster 0016+16 discussed below), the six other clusters with z > 0.3 all had Tg < 4 keV. Nearby clusters of similar X-ray luminosity have average temperatures of Tg 7 keV. Thus it appeared that gas temperatures in clusters might be increasing with time. Since the temperature of gas in a cluster reflects in part the depth of the cluster potential well (Section 5.5), this suggested that clusters might be growing through the merger of smaller clusters, in accord with models developed by Perrenod (1978b). White et al. (1981a) also detected a fairly distant cluster (SC2059-247) with a very high X-ray luminosity and a low X-ray temperature. However, White et al. (1987) observed a sample of 10 high redshift clusters; of the five detected cluster X-ray sources, all have reasonably high X-ray temperatures Tg 6 keV. They suspect that the previous evidence for low X-ray temperatures by Perrenod and Henry was an artifact of the small size of the sample being discussed. They also point out that some high luminosity, low temperature clusters, such as SC2059-247, may be extreme examples of clusters with cooling accretion flows at their centers.
Several of the high redshift clusters that have been observed as X-ray sources (Henry et al., 1979, 1982; Helfand et al., 1980) are Butcher-Oemler clusters (Butcher and Oemler, 1978a). As discussed in Section 2.10.2, these are high redshift clusters that appear to contain blue galaxies with colors similar to those of spirals in nearby clusters. Prior to the X-ray observations, the most straightforward explanation of these clusters was that they did not contain enough gas to strip spiral galaxies effectively by ram pressure ablation (Norman and Silk, 1979; Sarazin, 1979). Obviously, the detection of large quantities of hot gas in these clusters has made that explanation untenable. Recently optical observations indicate that the blue galaxies in these clusters are not normal spirals and are unlike any class of present day galaxies (Dressler and Gunn, 1982).
The most spectacular high redshift cluster to be detected so far is probably the z = 0.54 cluster 0016+16 (White et al., 1981b). Unfortunately, the cluster field also contains a foreground cluster at z = 0.30 (Ellis et al., 1985), which probably affected early estimates of the richness of the z = 0.54 cluster and of the colors of its galaxies (Koo, 1981). Assuming the emission is entirely due to the higher redshift cluster, 0016+16 has one of the highest X-ray luminosities observed Lx = 3 × 1045h50 - 2 erg/s, and may have a high temperature, although this is very uncertain. The cluster is probably about as rich as Coma (Ellis et al., 1985; Koo, 1981). The cluster 0016+16 may also be similar to nearby rich clusters in that it appears to have predominately red galaxies (it does not show the Butcher-Oemler effect), although the foreground cluster makes this conclusion somewhat uncertain. A microwave decrement (Section 4.5) of -1.4 mK has been detected from the cluster (Birkinshaw et al., 1981a). This effect has not been detected in many nearby clusters, and if the detection were confirmed it would demonstrate the extreme luminosity and temperature of 0016+16.
The tentative conclusion at this point is that the X-ray properties of clusters do not appear to evolve dramatically to redshifts of z 0.5, and that evolution of the X-ray properties is probably not the explanation of the Butcher-Oemler effect.