Actively Star-forming Elliptical Galaxies at Low Redshifts in the Sloan Digital Sky Survey

We report discovery of actively star forming elliptical galaxies in a morphologically classified sample of bright galaxies at a low redshift obtained from the Sloan Digital Sky Survey. The emission lines of these galaxies do not show the characteristics of active galactic nuclei, and thus their strong H$\alpha$ emission is ascribed to star formation with a rate nearly as high as that is seen in typical late spiral galaxies. This is taken as evidence against the traditional view that all elliptical galaxies formed early and now evolve only passively. The frequency of such star forming elliptical galaxies is a few tenths of a percent in the sample, but increases to 3% if we include active S0 galaxies. We may identify these galaxies as probable progenitors of so-called E+A galaxies that show the strong Balmer absorption feature of A stars superimposed on an old star population. The approximate match of the abundance of active elliptical plus S0 galaxies with that of E+A galaxies indicates that the duration of such late star formation episodes is of the order of $\gsim 1$ Gyr. If we interpret these galaxies as new additions to the early-type galaxy population, and if we assume a power law for their number evolution, the abundance of early-type galaxies at $z=1$ is about 30% less than that at $z=0$.Comment: Accepted for publication in Astrophys. J. Letter