  We present an analysis of star formation and nuclear activity of about 28000
galaxies in a volume-limited sample taken from SDSS DR4 low-redshift catalogue
(LRC) taken from the New York University Value Added Galaxy Catalogue
(NYU-VAGC) of Blanton et al. 2005, with 0.005<z<0.037, ~90\% complete to
M_r=-18.0. We find that in high-density regions ~70 per cent 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 per cent for Mr <~ -21 to zero by Mr ~ -18. Moreover the few
passively evolving dwarf galaxies in field regions appear as satellites to
bright (>~ L*) galaxies. Moreover the fraction of galaxies with the optical
signatures of an active galactic nucleus (AGN) decreases steadily from ~50\% at
Mr~-21 to ~0 per cent by Mr~-18 closely mirroring the luminosity dependence of
the passive galaxy fraction in low-density environments (see fig. 1 continuous
lines). This result reflects the increasing importance of AGN feedback with
galaxy mass for their evolution, such that the star formation histories of
massive galaxies are primarily determined by their past merger history.
