The Motions of Clusters and Groups 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 (Ω = 1) CDM, low-density CDM, HDM (Ω = 1), and PBI. We find that rich clusters of galaxies exhibit a Maxwellian distribution of peculiar velocities in all models, as expected from a Gaussian initial density fluctuation field. The clusters appear to be a fundamental and efficient tracer of the large-scale velocity-field. The cluster 3-D velocity distribution is generally similar in the models: it peaks at v ∼ 500 km s, and extends to high cluster velocities of v ∼ 1500 km s. Approximately 10% of all model rich clusters move with high peculiar velocities of v ≥ 10 km s. 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. In all but the low-density CDM model, the mass 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 low-density CDM model exhibits Maxwellian velocity distributions for clusters, for groups, and for matter that are all similar to each other. The model velocity distributions of groups and clusters of galaxies are compared with observations. The data are generally consistent with the models, but exhibit a somewhat larger high-velocity tail, to vr ∼ 3000 km s. While this high-velocity tail 2 is similar to the HDM model predictions, the data are consistent with the other models studied, including the low-density CDM model, which best fits most other large-scale structure observations. The observed velocity distribution is consistent with a Gaussian initial density fluctuation field. Subject headings: cosmology: theory – galaxies: clustering