Peculiar velocities of galaxy clusters

We investigate the peculiar velocities predicted for galaxy clusters by theories in the cold dark matter family. A widely used hypothesis identifies rich clusters with high peaks of a suitably smoothed version of the linear density fluctuation field. Their peculiar velocities are then obtained by extrapolating the similarly smoothed linear peculiar velocities at the positions 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. Furthermore, the mean peculiar velocity of this material agrees well with the linear value at the peak. The late-time growth of peculiar velocities is, however, systematically underestimated by linear theory. At the time when clusters are identified, we find their rms peculiar velocity to be about 40 per cent larger than predicted. Non-linear effects are particularly important in superclusters. These systematics must be borne in mind when using cluster peculiar velocities to estimate the parameter combination σ8Ω0.6.