Peculiar velocities of galaxy clusters

We investigate the peculiar velocities predicted for galaxy clusters by the ories in the cold dark matter family. A widely used hypothesis identifies r ich clusters with high peaks of a suitably smoothed version of the linear d ensity fluctuation field. Their peculiar velocities are then obtained by ex trapolating the similarly smoothed linear peculiar velocities at the positi ons of these peaks. We test these ideas using large high-resolution N-body simulations carried out within the Virgo supercomputing consortium. We find that at early times the barycentre of the material that ends up in a rich cluster is generally very close to a peak of the initial density field. Fur thermore, the mean peculiar velocity of this material agrees well with the linear value at the peak. The late-time growth of peculiar velocities is, h owever, systematically underestimated by linear theory. At the time when cl usters are identified, we find their rms peculiar velocity to be about 40 p er cent larger than predicted. Non-linear effects are particularly importan t in superclusters. These systematics must be borne in mind when using clus ter peculiar velocities to estimate the parameter combination sigma(8)Omega (0.6).