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

We consider the dynamical evolution of small-scale substructure in clu sters within two extreme alternate scenarios for their possible origin : (1) the accretion of groups (or small clusters) on quasi-radial orbi ts and (2) the merger of clusters of similar masses, followed by the d ecoupling of their dense cores. Using simple analytical arguments and checking with numerical computations, we show that objects are destroy ed by the tidal field of the global cluster potential if their mean de nsity 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. Si nce the cores of clusters are much denser than groups, they are consid erably more robust to tides, but the least massive are destroyed or se verely 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.