Moore et al. (1996) have pointed out that strong tidal interactions or galaxy ``harassment'' in the cluster environment play an important role in galaxy morphological evolution. In the simulations discussed here, most galaxies show signs of tidal disturbance over their history. Some major effects are the excitation of open spiral structures, warps and even tidal tails in close encounters of individual galaxies with the growing central BCG. Many galaxies are on orbits that take them within 100 kpc of the cluster center and the tidal fields within this radius are sufficient to obviously distort galaxies. Isolated disturbed galaxies seen in clusters may result in high speed encounters with the parent cluster's central potential. Higher resolution simulations of individual disks in orbit in fixed cluster potentials clearly demonstrate this effect (Dubinski & Hayes 1999). Tidal heating of disks can be strong in this environment and could account in part for the greater frequency of S0 galaxies in clusters as revealed by the morphology-density effect (Dressler 1980).
Galaxy-galaxy interactions also create disturbed galaxies but this occurs mainly in bound pairs or groups which eventually merge forming elliptical-like remnants. It appears that the general origin of elliptical galaxies in the cluster environment is the merging of sub-groups in the hierarchy. Moore et al. (1998) claim that galaxy harassment may also produce dwarf elliptical galaxies but to be definitive higher resolution is needed since numerical heating effects can be severe when simulating galaxies over a Hubble time even when using 100K particles. All of this work needs to be placed on a stronger, statistical foundation with more simulations and better resolution.
