The different physical mechanisms that drive the star formation histories of giant and dwarf galaxies

We present an analysis of star-formation and nuclear activity in galaxies as a function of both luminosity and environment in the fourth data release of the Sloan Digital Sky Survey (SDSS DR4). Using a sample of 27753 galaxies in the redshift range 0.005<z<0.037 that is �90 % complete to Mr = −18.0 we find that the EW(Hα) distribution is strongly bimodal, allowing galaxies to be robustly separated into passivelyevolving and star-forming populations about a value EW(Hα) = 2˚A. In high-density regions ∼70 % of galaxies are passively-evolving independent of luminosity. In the rarefied field however, the fraction of passively-evolving galaxies is a strong function of luminosity, dropping from 50 % for Mr �−21 to zero by Mr ∼−18. Indeed for the lowest luminosity range covered (−18<Mr <−16) none of the ∼600 galaxies in the lowest density quartile are passively-evolving. The few passively-evolving dwarf galaxies in field regions appear as satellites to bright (�L ∗ ) galaxies. We find a systematic reduction of ∼ 30 % in the Hα emission from dwarf (−19<Mr <−18) star-forming galaxies in high-density regions with respect to field values, implying that the bulk of

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