When did cores of clusters of galaxies form?

Recently there has been a rapid growth of observational evidence which indicates that most clusters of galaxies are at present composed of two dynamically distinct entities: a core of high central concentration which seems to be relaxed and virialized and an extended region of infalling matter on larger scales. In this paper, the usual procedure for determining the time of breakaway of these structures from the general expansion is improved by correcting the cosmological density contrast in an open universe for an arbitrary perturbation at its turnaround redshift Zt. An expression for Zt is derived in terms of observed properties, and its dependence on the model of collapse is discussed. 31 refs