VELOCITY-FIELDS AND ALIGNMENTS OF CLUSTERS IN GRAVITATIONAL-INSTABILITY SCENARIOS

The structure and evolution of the outskirts of clusters in several gr avitational instability scenarios are studied. By means of the Hoffman -Ribak constrained random field code we generate realizations of fluct uation fields containing protoclusters of a specified height and shape . The samples generated consist of 64(3) particles in a box with a siz e of 50 h-1 Mpc. By means of a P3M N-body code, using a 128(3) grid, t he evolution of the resulting particle distribution is followed into t he nonlinear regime. The protoclusters are 3sigma0 fluctuations [sigma 0 = sigma0(4h-1 Mpc)] in a cold dark matter scenario and in two scale- free scenarios [P(k) is-proportional-to k(n), n = 0 or -2], OMEGA0 = 1 . We find that power in the initial fluctuation spectrum on small scal es leads to the formation of substructure. The accretion of this subst ructure prevents the cluster from becoming as flattened as in the case of smooth ellipsoids. The shape of the clusters is derived from the i nertia tensor; the two axis ratios that it yields are approximately co nstant in time. The mass distribution on scales of a few megaparsecs h as a triaxial shape. Axis ratios typically vary between approximately 0.6 and 0.8. Despite the small changes in shape, the orientation of th e major axis of the cluster is heavily affected by the infall of small -scale structure. In general, the elongated cluster points in the dire ction from which the last subcluster fell into the core. Sometimes the orientation changes by as much as 70-degrees. These changes in orient ation cast doubt on the alignments of clusters that have been reported in the past. The presence of alignments is found to be consistent wit h a picture where the substructure falls into the cluster along a fila ment. The more isotropic the initial distribution of groups and small scale structure, the greater the changes in orientation of the major a xis of the cluster. We also find that all clusters have evolved signif icantly in the recent past. The accretion of small-scale structure sev erely disrupts the velocity field. As a result, techniques that use th e velocity field on scales of a few megaparsecs as a means of constrai ning OMEGA are severely hindered by the disruptions in the cluster hal o. These disruptions can persist for some time after the substructure has fallen into the cluster. The application of the spherical infall m odel to a smooth and apparently relaxed cluster may not result in the detection of the caustics. This provides a plausible explanation of wh y no caustics have been detected around the Coma Cluster.