4.3. Formation of Galaxy Clusters
Kaiser (1986) considers that clusters formed quite recently and that material beyond ~ 1.5h-1 - 2h-1 Mpc, may still be infalling. A test of this might be the detection of subclustering in the outer regions of rich galaxy clusters (West and Bothun, 1990).
The idea that galaxy clusters, and in particular their outer regions, may contain information about their past is an important one. West et al., (1988, 1989) have examined systematic properties of simulated clusters, in the hope of recognizing different initial conditions. They have covered a large part of the cosmological parameter space. They have looked, for example, at n = - 2 - 1, 0 and n = 0 pancake scenarios and they even do a universe with baryon = tot = 0.15 and n = 0. The novelty of the approach was to use low resolution simulations to locate clusters, followed by high resolution simulations to discover their properties. (An alternative approach would be to use Bertschinger's (1987) method of setting up initial conditions).
They look at density and velocity dispersion profiles, subclustering and cluster alignment. Perhaps unsurprisingly, they find that the central regions of galaxy clusters yield little information about initial conditions. The strongest tests of clustering theories lie in observing alignments of the clusters with each other and their surroundings, and in the amount of subclustering present in the outer regions. Simulations with dark matter show a rapid segregation of light and dark matter, which causes a systemic change of derived mass to light ratio with appealing to any bias mechanism. The subject is well reviewed by West (1989).
The role of dynamical friction during the cluster formation process was considered by Carlberg, Couchman and Thomas (1990) and by Carlberg and Duninski (1991). They argue that the effect of dynamical friction is to lower the velocity dispersion of the galaxies in a cluster relative to the velocity dispersion of the collisionless dark matter. The ratio of the velocity dispersions is called by them the velocity bias. The reduced velocity dispersion gives rise to a steeper light profile in clusters.
Evrard (1989) argues that the CDM theory has a problem in generating clusters of galaxies having velocity dispersion in excess of ~ 1000 km s-1, of which there are several examples. Peebles, Daly and Juskiewicz (1989) amplify this. These claims are countered by Frenk et al. (1990) saying that there is an observational problem in knowing what the distribution of velocity dispersions of rich clusters of galaxies really is. There is even the suggestion that estimates of the velocity dispersions of such distant galaxy clusters may be hampered by projection effects.
There has been a lot of work done on the origin of the morphological types of galaxies in different environments and how these are modified during the evolution of the cluster via mergers, gas stripping and other processes. (See, for one random example, Evrard, Silk and Szalay (1990)).