The rise and fall of satellites in galaxy clusters

We use N-body simulations to study the infall of dark matter haloes onto rich clusters of galaxies. After identification of all cluster progenitors in the simulations, we select those haloes which accrete directly onto the main cluster progenitor. We construct the mass function of these merging \ satellites, and calculate the main orbital parameters for the accreted lumps. The average eccentricity of the orbits is \ \ 0. 5, while either radial or almost circular orbits are equally avoided. More massive satellites move along slightly more eccentric orbits, with lower specific angular momentum and a smaller pericentre. We find that the infall of satellites onto the main cluster progenitor has a very anisotropic distribution. This anisotropy is to a large extent responsible for the shape and orientation of the final cluster, and to a lesser extent for that of its velocity ellipsoid. At the end of the simulations, the major axis of the cluster is aligned both with that of its velocity ellipsoid, and with the major axis of the ellipsoid defined by the satellite infall pattern, to better than 30\ on average. We also find that, in lower mass clusters, a higher fraction of the final virial mass is provided by small, dense satellites. These sink to the centre of the parent cluster and so enhance its central density. This mechanism is found to be partially responsible for the correlation between halo masses and characteristic overdensities, recently highlighted by Navarro, Frenk \\& White (1996).