PECULIAR VELOCITIES OF GALAXY CLUSTERS - A COMPARISON OF MIXED DARK-MATTER AND LOW-DENSITY COLD DARK-MATTER

We determine the distribution of peculiar velocities of galaxy cluster s in two competing cosmological scenarios: a spatially flat, low-densi ty, cold dark matter universe with cosmological density parameter OMEG A = 0.2 and a cosmological constant LAMBDA = 3H-0(2)(1 - OMEGA) (LCDM) ; and an OMEGA = 1 universe containing a mixture of hot and cold dark matter (MDM). Although the spatial clustering properties of clusters i n the two models are almost identical, the rms cluster peculiar veloci ties in the MDM universe are substantially higher. We compare the theo retical peculiar velocity distributions with observations and find tha t both models have difficulties in reproducing the observed distributi on of cluster peculiar velocities. The reliable detection of several m ore clusters with peculiar velocities greater than 1000 km s-1 would b e difficult to reconcile with the models, particularly LCDM.