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.