5.10. The origin and evolution of the intracluster medium
Where does all of this X-ray emitting intracluster gas come from? Did it fall into clusters from intergalactic space, or was it ejected from galaxies? When did it first fill the great volumes of space between galaxies in clusters? What can we learn about the origin of galaxies and clusters from the intracluster gas? Theories of the origin of the intracluster medium attempt to explain the current properties of X-ray clusters, to make predictions about their past history that can be tested with observations of clusters at high redshift, and to relate the history of the intracluster gas to theories of the origin of structure in the universe.
There are two basic constraints on such theories from the observations of present day X-ray clusters. First, these clusters typically contain 1014 M of intracluster gas, an amount at least comparable to the mass of luminous material in galaxies and at least one-tenth of the total mass of the cluster. Second, this gas produces X-ray lines from iron which require that the abundance of that element be about one-half of its solar value if the gas is homogeneous.
At present, there are very few observations of high redshift X-ray clusters, and thus the constraints that can be imposed on theories of the evolution of X-ray clusters by these observations are fairly weak. The only safe statement one can make at the present time is that X-ray clusters do not show any evidence of very dramatic evolution out to redshifts of z 0.5 (Section 4.8). Another constraint on the evolution of X-ray clusters comes from the diffuse X-ray background (Fabian and Nulsen, 1979); the total emission from high redshift clusters cannot exceed the background brightness.
The question of the origin of the intracluster gas is strongly tied to the question of the origin of clusters and galaxies. This is a major area of astronomical research, and I shall not even attempt to summarize the theories in this area. Instead, I shall deal very narrowly with theories of the intracluster medium and shall largely ignore their dependence on models for galaxy formation, except when the X-ray cluster models provide information that affects these theories. Discussions of the effects that X-ray cluster observations have on theories of galaxy formation include Silk (1978), Binney and Silk (1978), White and Rees (1978), Fabian and Nulsen (1979), Binney (1980), and Field (1980).
There is an increasingly large amount of observational information on the evolution of galaxies; here only a few points will be noted. First, galaxies contain stars with a range of ages and heavy element abundances. Elliptical and S0 galaxies contain mainly older stars, with ages of around 1010 yr (Fall, 1981). Although galaxies do contain some stars with very low heavy element abundances, there is no significant population of stars with no heavy elements, and the number of low abundance stars is less than might be expected if all the heavy elements were formed in stars like the present stellar population (Carr et al., 1984). No mechanism is known to produce heavy elements in any quantity outside of stars. This has led to the suggestion that there was an early (before 1010 yr ago) generation of stars, which produced the minimum level of heavy elements seen in stars today (Carr et al., 1984). Second, there is some evidence that the population of galaxies has evolved in some clusters since redshifts of z 0.5; this is the 'Butcher-Oemler effect' (Section 2.10.2). These clusters have an excess of blue galaxies, which suggests that they were undergoing fairly rapid star formation at that time. Finally, violent activity in the nuclei of galaxies appears to have been much more common in the past (Schmidt, 1978); this activity produces Seyfert galaxies, radio galaxies, and quasars. There were many more very luminous quasars at a time corresponding to a redshift of 2 than are seen around us today. On the other hand, since few quasars are known with redshifts greater than 3.5 (Osmer, 1978), something may have occurred then to start the violent nuclear activity in galaxies. Also, if quasars are all located in the nuclei of galaxies, then galaxies must have formed by a redshift of 3. Although this nuclear activity in galaxies is very poorly understood, most current theories require that gas be supplied to the nucleus (Rees, 1978); thus its variation may be related to the supply of intracluster gas and the stripping of gas from galaxies.