The Different Environmental Dependencies of Star Formation for Giant and Dwarf Galaxies

We examine the origins of the bimodality observed in the global properties ofgalaxies around a stellar mass of 3x10^10 M_sun by comparing the environmentaldependencies of star-formation for the giant and dwarf galaxy populations. TheSloan Digital Sky Survey DR4 spectroscopic dataset is used to produce a sampleof galaxies in the vicinity of the supercluster centered on the cluster A2199at z=0.03 that is ~90% complete to a magnitude limit of M*+3.3. From these wemeasure global trends with environment for both giant (M_r<-20 mag) and dwarf(-197 Gyr) or passive (EW[H_alpha]<4A) falls gradually from ~80% in the cluster cores to ~40% in field regionsbeyond 3-4 R_virial, as found in previous studies. In contrast, we find thatthe dwarf galaxy population shows a sharp transition at ~1 R_virial, from beingpredominantly old/passive within the cluster, to outside where virtually allgalaxies are forming stars and old/passive galaxies are only found assatellites to more massive galaxies. These results imply fundamentaldifferences in the evolution of giant and dwarf galaxies: whereas thestar-formation histories of giant galaxies are determined primarily by theirmerger history, star-formation in dwarf galaxies is much more resilient to theeffects of major mergers. Instead dwarf galaxies become passive only once theybecome satellites within a more massive halo, by losing their halo gasreservoir to the host halo, or through other environment-related processes suchas galaxy harassment and/or ram-pressure stripping.Comment: 4 pages, 4 figures, accepted for publication in ApJ