Environmental Effects on Galaxy Evolution
Tomotsugu GOTO
It is a remarkable feature that properties of galaxies vary according to their environments. However, only little has been known on the origin of this variety in galaxies. Using the Sloan Digital Sky Survey (SDSS) data, we investigate the morphology-density relation to find two characteristic environment where the morphology-density relation abruptly changes (0.3 and 2 Rvir).The result has significant implication for the underlying physical mechanism since it suggests that two different mechanisms might be responsible for the morphology-density relation. In addition, we study the environment of unusual spiral galaxies with no [OII] or Halpha emission lines (passive spirals), and find that passive spiral galaxies preferentially live in cluster infalling regions (1-10 Rvir}) Thus, the origin of passive spiral galaxies is likely to be cluster related. In contrast, we find that E+A (post-starburst) galaxies, which have been believed to be a product of cluster environment for a long time, have little to do with cluster environment, but have their origin in merger/interaction with accompanying galaxies. Based on all of our observational results, we propose possible fate of cluster infalling galaxies.