Abstract

We employ spectroscopic and photometric data from the Sloan Digital Sky Survey Data Release 7, in a 500-deg2 region, to understand the evolution of dwarf [similar to(M* + 2) M-z M* + 4] galaxies in the Coma supercluster (z = 0.023). We show that in the Coma supercluster, the red dwarf galaxies are mostly concentrated in the dense cores of the Coma and Abell 1367 clusters, and in the galaxy groups embedded in the filament connecting them. The post-starburst (k+A) dwarfs, however, are found in the infall regions of the Coma and Abell 1367 clusters, and occasionally in galaxy groups embedded along the filament, suggesting that strong velocity fields prevalent in the vicinity of deep potential wells may be closely related to the mechanism(s) leading to the post-starburst phase in dwarf galaxies. Moreover, the blue colour of some k+A dwarfs in the Coma cluster, found within its virial radius, suggests that the star formation in these galaxies was quenched very rapidly in the last 500 Myr. More than 60 per cent of all red dwarf galaxies in the supercluster have 0-3 A of H delta in absorption, which suggests that a major episode of star formation occurred in a non-negligible fraction of these galaxies, ending within the last Gyr, allowing them to move to the red sequence. The distribution of the blue dwarf galaxies in the Coma supercluster is bimodal in the EW(H alpha)-EW(H delta) plane, where EW represents equivalent width, with one population having very high emission in H alpha and some emission in H delta. A subpopulation of blue dwarfs is coincident with the red dwarfs in the EW(H alpha)-EW(H delta) plane, showing absorption in H delta and relatively lower emission in H alpha. We suggest that a large fraction of the latter population represents the progenitors of the passive dwarf galaxies that are abundantly found in the cores of low-redshift rich clusters, such as Coma.