WHEN DID CORES OF CLUSTERS OF GALAXIES FORM

There is now much observational evidence which indicates that the formation of clusters of galaxies is still an ongoing process. However, in the central region of clusters, there often resides a dense and relaxed component which appears to have formed earlier than its surroundings. Assuming the gravitational instability picture, what is the place of these structures in the sequence of creation? We improve the usual procedure for determining the time of breaking away from the general expansion, and show that the redshift of turnaround, Zt, depends on a single parameter α which combines directly observed quantities: recession velocity, central velocity dispersion, and angular size of the core. The developed estimator is totally independent of the value of H0. However, it involves a numerical coefficient which varies with the model of collapse. Thus, even with complete and exact observational data, our lack of knowledge of the correct collapse model prevents Zt from being sharply determined. Results are obtained for structures with various properties and for various choices of the cosmological density parameter Ω0. A typical case is the central component of the Coma Cluster, for which we get 4.1 ≤ Zt < 8.5 for Ω0 = 0.2 or 2.4 < Zt < 5.0 for Ω0 = 1. It also turns out that unless protoclusters have collapsed in substantially different ways, the epoch of protocluster turnaround is correlated with the velocity dispersion in the cluster center. The existence of highly nonlinear mass perturbations of the order of 1013.5±0.5 M⊙ at such moderate redshifts might have had considerable influence on galaxy evolution.