  We study the joint effect of tidal torques and dynamical friction on the
collapse of density peaks solving numerically the equations of motion of a
shell of barionic matter falling into the central regions of a cluster of
galaxies. We calculate the evolution of the expansion parameter, a(t), of the
perturbation using a coefficient of dynamical friction eta_{cl} obtained from a
clustered system and taking into account the gravitational interaction of the
quadrupole moment of the system with the tidal field of the matter of the
neighboring proto-galaxies. We show that within high-density environments, such
as rich clusters of galaxies, tidal torques and dynamical friction slow down
the collapse of low-nu peaks producing an observable variation of the parameter
of expansion of the shell. As a consequence a bias of dynamical nature arises
because high-density peaks preferentially collapse to form halos within which
visible objects eventually will condense. For a standard Cold Dark Matter model
this dynamical bias can account for a substantial part of the total bias
required by observations on cluster scales.
