THE DYNAMICAL SURVIVAL OF SMALL-SCALE SUBSTRUCTURE IN RELAXED GALAXY CLUSTERS

We consider the dynamical evolution of small-scale substructure in clusters within two extreme alternate scenarios for their possible origin: (1) the accretion of groups (or small clusters) on quasi-radial orbits and (2) the merger of clusters of similar masses, followed by the decoupling of their dense cores. Using simple analytical arguments and checking with numerical computations, we show that objects are destroyed by the tidal field of the global cluster potential if their mean density is small compared to the mean cluster density within the radius of closest approach of the group or detached core. Accreted groups and small clusters are thus tidally disrupted in one cluster crossing. Since the cores of clusters are much denser than groups, they are considerably more robust to tides, but the least massive are destroyed or severely stripped by tides, while the others are brought to the cluster center by dynamical friction (and subsequently merge) in less than one orbit. The longest lived substructures are detached cores, roughly 10 times less massive than the cluster, starting in near-circular orbits beyond 1 h-1 Mpc from the cluster center.