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7.2.3. Clusterin properties

It is well known that ellipticals occur preferentially in high density regions such as the cores of rich galaxy clusters. However, Eq. (7.3) and Figure 7.1 show that merging will be relatively rare in such an environment since the average encounter velocity (v gtapprox 1000 km/sec) is much larger than the typical velocity dispersion of the constituent galaxies. The cosmological N-body experiments, on the other hand, all show a good correlation between the fraction of merged galaxies and density contrast, in qualitative agreement with observations. This arises because, as White (1976) has already illustrated, a rich cluster will form by the amalgamation of low-velocity dispersion subclumps. Since the merger rate is strongly density dependent, the merger probability is higher in the past, hence it is plausible that merging may be fairly complete in subclumps, before a rich cluster relaxes. Of course, the merger rate must drop rapidly once the cluster reaches virial equilibrium. In the cosmological simulations at least the effect appears strong enough to produce a correlation between merger fraction and density contrast.