Morphological Transformation from Galaxy Harassment

Galaxy morphologies in clusters have undergone a remarkable transition overthe past several billion years. Distant clusters at $z \sim 0.4$ are filledwith small spiral galaxies, many of which are disturbed and show evidence ofmultiple bursts of star-formation. This population is absent from nearbyclusters where spheroidals comprise the faint end of the luminosity function.Our numerical simulations follow the evolution of disk galaxies in a richcluster owing to encounters with brighter galaxies and the cluster's tidalfield---galaxy harassment. After a bursting transient phase, they undergo acomplete morphological transformation from "disks" to"spheroidals". We examinethe remnants and find support for our theory in detailed comparisons of thephotometry and kinematics of the spheroidal galaxies in clusters. Our modelnaturally accounts for the intermediate age stellar population seen in thesespheroidals as well as the trend in dwarf to giant ratio with cluster richness.The final shapes are typically prolate and are flattened primarily by velocityanisotropy. Their mass to light ratios are in the range 3---8 in good agreementwith observations.Comment: Final version, to be published in the Ap.J. 1998, video edition with the "Galaxy Harassment" movi