THE MOTIONS OF CLUSTERS AND GROUP OF GALAXIES

The distributions of peculiar velocities of rich clusters and of group s of galaxies are investigated for different cosmological models and a re compared with observations. Four cosmological models are studied: s tandard cold dark matter (CDM) (Omega = 1); low-density CDM (Omega = 0 .3); hot dark matter (HDM) (Omega = 1); and primeval baryonic isocurva ture (PBI) (Omega = 0.3). All models are normalized to the microwave b ackground fluctuations observed by COBE. We find that rich clusters of galaxies exhibit a Maxwellian distribution of peculiar velocities in all models, as expected from a Gaussian initial density field. The clu sters appear to be fundamental and efficient tracers of the large-scal e velocity field. The cluster three-dimensional velocity distribution typically peaks at upsilon similar to 600 km s(-1) and extends to high cluster velocities of upsilon similar to 2000 km s(-1). The low-densi ty CDM model exhibits somewhat lower velocities: it peaks at similar t o 400 km s(-1) and extends to similar to 1200 km s(-1). Approximately 10% (similar to 1% for low-density CDM) of all model rich clusters mov e with high peculiar velocities of upsilon greater than or equal to 10 (3) km s(-1). The highest velocity clusters frequently originate in de nse superclusters. The model velocity distributions of rich clusters a re compared with the model velocity distributions of small groups of g alaxies, and of the total matter. The group velocity distribution is, in general, similar to the velocity distribution of the rich clusters. The matter velocity distribution is similar to that of the rich clust ers for the Omega = 0.3 models; these models exhibit Maxwellian veloci ty distributions for clusters, for groups, and for matter that are all similar to one another. The mass distribution in the Omega = 1 models , however, exhibits a longer tail of high velocities than do the clust ers. This high-velocity tail originates mostly from the high velocitie s that exist within rich clusters. The model velocity distributions of groups and clusters of galaxies are compared with observations. The d ata exhibit a larger high-velocity tail, to upsilon(r) greater than or equal to 2000 km s(-1), than is seen in the model simulations (except for HDM). Because of the large observational uncertainties, however, the data are consistent at a similar to 1-3 sigma level with the model predictions and with a Gaussian initial density field. Accurate obser vations of cluster peculiar velocities, especially at the high-velocit y tail, should provide powerful constraints on the cosmological models .