NEW IMAGES OF THE DISTANT, RICH CLUSTER CL-0939+4713 WITH WFPC2

New images of the distant cluster of galaxies CL 0939 + 4713 at z = 0.41 have been taken with the WFPC2 on the HST. The images go deeper and show considerably more detail than the aberrated images from WFPC1. We confirm that for this cluster the Butcher-Oemler effect results from a surprisingly large population of late-type spiral and irregular galaxies. We also confirm and find further evidence for mergers and interactions that are a probable mechanism for the disappearance of this population by the present epoch. Based on the irregular forms of spirals in this cluster, we suggest that another mechanism may be the destabilization of disks in cluster galaxies as their halos are tidally distorted or removed through gravitational encounters. The new data reveal many more faint objects, with much finer detail, around a quasar at z = 2.055 (Dressler et al. 1992). These data demonstrate the remarkable ability of HST to study the structural evolution of galaxies to very high redshifts.