THE MORPHOLOGY OF DISTANT CLUSTER GALAXIES .2. HST OBSERVATIONS OF 4 RICH CLUSTERS AT Z-SIMILAR-OR-EQUAL-TO-0.4

We present Hubble Space Telescope (HST) WF/PC and WFPC2 observations o f four rich clusters of galaxies at redshifts z similar to 0.4: CL 144 7+2619, CL 0939+4713, CL 0024+1654, and Abell 370. Extensive ground-ba sed observations have shown that all have substantial populations of b lue, star-forming, and (in some cases) starbursting galaxies. We find the galaxy populations in these clusters to be approximately normal, i n that most galaxies can be accommodated in the z = 0 Hubble sequence and have the luminosities, colors, and distributions within the cluste rs that are expected for their Hubble types. However, in every cluster environment the galaxy populations contain a uniformly higher proport ion of late-type galaxies than is found in similar environments today. Confirming previous ground-based and HST observations, we find a rema rkably large fraction of disturbed systems. The appearance of many of these is suggestive of merging and tidally interacting objects, but se veral difficulties with this suggest that the newly described process of ''galaxy harassment'' may be a more likely explanation. These objec ts are clearly responsible for the majority of the starburst spectra d etected in ground-based observations of the clusters. They cannot, how ever, account for most of the large population of blue galaxies. The f raction of such disturbed galaxies varies from cluster to cluster in c oncert with the number of blue and of spiral galaxies. All these varia tions are well correlated with the structure and density of the cluste rs, but not with cluster richness. Close examination of the population of ''normal'' spirals shows that a significant fraction have very abn ormal patterns of star formation: Rings of star formation are much mor e common than two-armed spirals. Galaxies of this type are very rare i n nearby populations but also have been found in HST observations of h igh-redshift field populations. We speculate that these forms might be products of the cluster environment or may simply represent the norm for the earlier stage of disk evolution observed at this redshift.