Tidal Effects in Clusters of Galaxies

High-redshift clusters of galaxies show an over-abundance of spirals by afactor of 2-3, and the corresponding under-abundance of S0 galaxies, relativeto the nearby clusters. This morphological evolution can be explained by tidalinteractions with neighboring galaxies and with the hierarchically growingcluster halo. The efficiency of tidal interactions depends on the size andstructure of the cluster, as well as on the epoch of its formation. I simulatethe formation and evolution of Virgo-type clusters in three cosmologies: acritical density model Omega_0=1, an open model Omega_0=0.4, and a flat modelOmega_0=0.4 with a cosmological constant. The orbits of identified halos aretraced with a high temporal resolution (~10^7 yr). Halos with low relativevelocities merge only shortly after entering the cluster; after virializationmergers are suppressed. The dynamical evolution of galaxies is determined bythe tidal field along their trajectories. The maxima of the tidal force do notalways correspond to closest approach to the cluster center. They are producedto a large extent by the local density structures, such as the massive galaxiesand the unvirialized remnants of infalling groups of galaxies. Collisions ofgalaxies are intensified by the substructure, with about 10 encounters within10 kpc per galaxy in the Hubble time. These very close encounters add animportant amount (10-50%) of the total heating rate. The integrated effect oftidal interactions is insufficient to transform a spiral galaxy into anelliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger inthe low Omega_0 clusters