On the origins of starburst and poststarburst galaxies in nearby clusters

Nubble Space Telescope (NST) Wide Field Planetary Camera 2 images in B (F45 0W) and I (F814W) have been obtained for three starburst (SB) and two posts tarburst (PSB) galaxies in the Coma Cluster and for three such galaxies in the cluster DC 048-52. V (F555W) and I images for an additional PSB galaxy in Coma have been extracted from the HST archive. Six of these galaxies wer e previously classified as E/S0 on the basis of ground-based images, two as Sa, and the other as an irregular. The HST images reveal these SB/PSB galaxies to be heterogeneous in morpholo gy. Nevertheless, a common theme is that many of them, especially the SE ga laxies, tend to have centralized spiral structure that appears simply as a bright "bulge" on ground-based images. In addition, while some PSB galaxies exhibit distinct spiral structure, on the whole they have smoother morphol ogies than the SE galaxies. The morphologies and luminosity profiles are ge nerally consistent with substantial starbursts, in the form of centralized spiral structure (the SE galaxies), that fade into smoother morphologies (t he PSB galaxies), with lingering spectroscopic evidence for past central st arbursts. An important point is that the PSB galaxies retain disks; i.e., t hey have not evolved into spheroidal systems. While the morphologies revealed in the HST images are heterogeneous, and th us may not fit well into a single picture, we see evidence in several cases that the morphologies and centralized star formation have been driven by e xternal tidal perturbations. We discuss several physical mechanisms for ind ucing star formation in cluster galaxies with a view toward explaining the particular morphologies seen in the HST images.