  In hierarchical clustering, galaxy clusters accrete mass through the
aggregation of smaller systems. Thus, the velocity field of the infall regions
of clusters contains significant random motion superimposed on radial infall.
Because the purely spherical infall model does not predict the amplitude of the
velocity field correctly, methods estimating the cosmological density parameter
Omega_0 based on this model yield unreliable biased results. In fact, the
amplitude of the velocity field depends on local dynamics and only very weakly
on the global properties of the universe. We use N-body simulations of flat and
open universes to show that the amplitude of the velocity field of the infall
regions of dark matter halos is a direct measure of the escape velocity within
these regions. We can use this amplitude to estimate the mass of dark matter
halos within a few megaparsecs from the halo center. In this region dynamical
equilibrium assumptions do not hold. The method yields a mass estimate with
better than 30% accuracy. If galaxies trace the velocity field of the infall
regions of clusters reliably, this method provides a straightforward way to
estimate the amount of mass surrounding rich galaxy clusters from redshift data
alone.
