Velocity Fields and Alignments of Clusters in Gravitational Instability Scenarios

Abstract

The structure and evolution of the outskirts of clusters in several gravitational instability scenarios are studied. By means of theHoffman-Ribak constrained random field code we generate realizations offluctuation fields containing protoclusters of a specified height andshape. The samples generated consist of 643 particles in abox with a size of 50 h-1 Mpc. By means of a P3MN-body code, using a 1283 grid, the evolution of theresulting particle distribution is followed into the nonlinear regime.The protoclusters are 3σ0 fluctuations[σ0 = σ0(4 h-1 Mpc)] in acold 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 scalesleads to the formation of substructure. The accretion of thissubstructure prevents the cluster from becoming as flattened as in thecase of smooth ellipsoids. The shape of the clusters is derived from theinertia tensor; the two axis ratios that it yields are approximatelyconstant in time. The mass distribution on scales of a few megaparsecshas a triaxial shape. Axis ratios typically vary between ˜0.6 and0.8.Despite the small changes in shape, the orientation of the major axis ofthe cluster is heavily affected by the infall of small-scale structure.In general, the elongated cluster points in the direction from which thelast subcluster fell into the core. Sometimes the orientation changes byas much as 70°. These changes in orientation cast doubt on thealignments of clusters that have been reported in the past. The presenceof alignments is found to be consistent with a picture where thesubstructure falls into the cluster along a filament. The more isotropicthe initial distribution of groups and small scale structure, thegreater the changes in orientation of the major axis of the cluster.We also find that all clusters have evolved significantly in the recentpast. The accretion of small-scale structure severely disrupts thevelocity field. As a result, techniques that use the velocity field onscales of a few megaparsecs as a means of constraining Ω areseverely hindered by the disruptions in the cluster halo. Thesedisruptions can persist for some time after the substructure has falleninto the cluster. The application of the spherical infall model to asmooth and apparently relaxed cluster may not result in the detection ofthe caustics. This provides a plausible explanation of why no causticshave been detected around the Coma Cluster.