THE RISE AND FALL OF SATELLITES IN GALAXY CLUSTERS

We use N-body simulations to study the infall of dark matter haloes on to rich clusters of galaxies. After identification of all cluster pro genitors in the simulations, we select those haloes that accrete direc tly on to 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 circularity of the orbits is epsi lon similar or equal to 0.5, while either radial or almost circular or bits are equally avoided. More massive, satellites move along slightly more eccentric orbits, with lower specific angular momentum and a sma ller pericentre. We find that the infall of satellites on to the main cluster progenitor has a very anisotropic distribution. This anisotrop y is to a large extent responsible for the shape and orientation of th e final cluster and of its velocity ellipsoid. At the end of the simul ations, the major axis of the cluster is aligned both with that of its velocity ellipsoid and with the major axis of the ellipsoid defined b y the satellite infall pattern, to approximate to 30 degrees on averag e. 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 correlat ion between halo masses and characteristic overdensities, recently hig hlighted by Navarro, Frenk & White.