The rise and fall of satellites in galaxy clusters

We use N-body simulations to study the infall of dark matter haloes onto richclusters of galaxies. After identification of all cluster progenitors in thesimulations, we select those haloes which accrete directly onto the maincluster progenitor. We construct the mass function of these merging satellites,and calculate the main orbital parameters for the accreted lumps. The averagecircularity of the orbits is epsilon = 0.5, while either radial or almostcircular orbits are equally avoided. More massive satellites move alongslightly more eccentric orbits, with lower specific angular momentum and asmaller pericentre. We find that the infall of satellites onto the main clusterprogenitor has a very anisotropic distribution. This anisotropy is to a largeextent responsible for the shape and orientation of the final cluster and ofits velocity ellipsoid. At the end of the simulations, the major axis of thecluster is aligned both with that of its velocity ellipsoid, and with the majoraxis of the ellipsoid defined by the satellite infall pattern, to 30 degrees onaverage. We also find that, in lower mass clusters, a higher fraction of thefinal virial mass is provided by small, dense satellites. These sink to thecentre of the parent cluster and so enhance its central density. This mechanismis found to be partially responsible for the correlation between halo massesand characteristic overdensities, recently highlighted by Navarro, Frenk &White (1996).Comment: Revised to match the published versio