Dynamics of Galaxies in Clusters

Recent observations by the Hubble Space Telescope show that spirals make up to 50% of the galaxies in clusters at redshift z ∼ 0.5 (Dressler et al. 1997; Ellis 1998). Spectroscopic evidence suggests bursts of recent, or even current, star formation. This picture is drastically different from the nearby clusters, populated mostly by featureless elliptical or S0 galaxies. The change is due to the evolution of S0s, which are depleted by about a factor of two at z ∼ 0.5. Observations suggest that star formation has ceased in the ellipticals by an epoch z ∼ 0.6, but continued in the S0s until at least z ∼ 0.3 (van Dokkum et al. 1998). It is apparent that a strong dynamical and morphological evolution of disk galaxies has occurred in clusters of galaxies.