ON THE DYNAMICAL ORIGIN OF BIAS IN CLUSTERS OF GALAXIES

We study the effect of the dynamical friction induced by the presence of substructure on the statistics of the collapse of density peaks. Ap plying the results of a recent paper by Antonuccio-Delogu & Colafrance sco (1994), we show that within high-density environments, such as ric h clusters of galaxies, the collapse of the low-v peaks is strongly de layed until very late epochs. A bias of dynamical nature thus naturall y arises because high-density peaks preferentially collapse to form ha los within which visible objects eventually will condense. We then der ive a selection function for these collapsing structures. Using this p hysical selection mechanism, we can calculate the values of the bias c oefficient on cluster scales for any hierarchical clustering scenario. For a standard cold dark matter model, we show here that the dynamica l bias that we derive can account for a substantial part of the total bias required by observations on cluster scales.