The motions of clusters and group of galaxies

The distributions of peculiar velocities of rich clusters and of groups of galaxies are investigated for different cosmological models and are compared with observations. Four cosmological models are studied: standard cold dark matter (CDM) (Ω = 1); low-density CDM (Ω = 0.3); hot dark matter (HDM) (Ω = 1); and primeval baryonic isocurvature (PBI) (Ω = 0.3). All models are normalized to the microwave background 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 clusters appear to be fundamental and efficient tracers of the large-scale velocity field. The cluster three-dimensional velocity distribution typically peaks at v ∼ 600 km s^-1 and extends to high cluster velocities of v ∼ 2000 km s^-1. The low-density CDM model exhibits somewhat lower velocities: it peaks at ∼400 km s^-1 and extends to ∼1200 km s^-1. Approximately 10% (∼1% for low-density CDM) of all model rich clusters move with high peculiar velocities of v ≥ 10³ km s^-1. The highest velocity clusters frequently originate in dense superclusters. The model velocity distributions of rich clusters are compared with the model velocity distributions of small groups of galaxies, 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 clusters for the Ω = 0.3 models; these models exhibit Maxwellian velocity distributions for clusters, for groups, and for matter that are all similar to one another. The mass distribution in the Ω = 1 models, however, exhibits a longer tail of high velocities than do the clusters. This high-velocity tail originates mostly from the high velocities that exist within rich clusters. The model velocity distributions of groups and clusters of galaxies are compared with observations. The data exhibit a larger high-velocity tail, to v_r ≥ 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 ∼1-3 σ level with the model predictions and with a Gaussian initial density field. Accurate observations of cluster peculiar velocities, especially at the high-velocity tail, should provide powerful constraints on the cosmological models.