The Different Environmental Dependencies of Star-formation for Giant and Dwarf Galaxies

We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3×10M⊙ by comparing the environmental dependencies of star-formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic dataset is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z = 0.03 that is &90% complete to a magnitude limit of M∗r + 3.3. From these we measure global trends with environment for both giant (Mr <−20 mag) and dwarf (−19 < Mr <−17.8 mag) subsamples using the luminosity-weighted mean stellar age and Hα emission as independent measures of star-formation history. The fraction of giant galaxies classed as old (t >7 Gyr) or passive (EW[Hα] ≤ 4 Å) falls gradually from &80% in the cluster cores to ∼40% in field regions beyond 3–4 Rvir, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ∼1Rvir, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences in the evolution of giant and dwarf galaxies: whereas the star-formation histories of giant galaxies are determined primarily by their merger history, star-formation in dwarf galaxies is much more resilient to the effects of major mergers. Instead dwarf galaxies become passive only once they become satellites within a more massive halo, by losing their halo gas reservoir to the host halo, or through other environment-related processes such as galaxy harassment and/or ram-pressure stripping. Subject headings: galaxies: clusters: general — galaxies: evolution — galaxies: stellar content