Velocity Fields and Alignments of Clusters in Gravitational Instability Scenarios

The structure and evolution of the outskirts of clusters in several gravitational instability scenarios are studied. By means of the Hoffman-Ribak constrained random field code we generate realizations of fluctuation fields containing protoclusters of a specified height and shape. The samples generated consist of 643 particles in a box with a size of 50 h-1 Mpc. By means of a P3M N-body code, using a 1283 grid, the evolution of the resulting particle distribution is followed into the nonlinear regime. The protoclusters are 3σ0 fluctuations σ0 = σ0(4 h-1 Mpc) in a cold dark matter scenario and in two scale-free scenarios P(k) ∝kn, n = 0 or -2, Ω0 = 1. We find that power in the initial fluctuation spectrum on small scales leads to the formation of substructure. The accretion of this substructure prevents the cluster from becoming as flattened as in the case of smooth ellipsoids. The shape of the clusters is derived from the inertia tensor; the two axis ratios that it yields are approximately constant in time. The mass distribution on scales of a few megaparsecs has a triaxial shape. Axis ratios typically vary between ∼0.6 and 0.8. Despite the small changes in shape, the orientation of the major axis of the cluster is heavily affected by the infall of small-scale structure. In general, the elongated cluster points in the direction from which the last subcluster fell into the core. Sometimes the orientation changes by as much as 70°. These changes in orientation cast doubt on the alignments of clusters that have been reported in the past. The presence of alignments is found to be consistent with a picture where the substructure falls into the cluster along a filament. The more isotropic the initial distribution of groups and small scale structure, the greater the changes in orientation of the major axis of the cluster. We also find that all clusters have evolved significantly in the recent past. The accretion of small-scale structure severely disrupts the velocity field. As a result, techniques that use the velocity field on scales of a few megaparsecs as a means of constraining Ω are severely hindered by the disruptions in the cluster halo. These disruptions can persist for some time after the substructure has fallen into the cluster. The application of the spherical infall model to a smooth and apparently relaxed cluster may not result in the detection of the caustics. This provides a plausible explanation of why no caustics have been detected around the Coma Cluster.