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

We report discovery of actively star forming elliptical galaxies in amorphologically classified sample of bright galaxies at a low redshift obtainedfrom the Sloan Digital Sky Survey. The emission lines of these galaxies do notshow the characteristics of active galactic nuclei, and thus their strongH$\alpha$ emission is ascribed to star formation with a rate nearly as high asthat is seen in typical late spiral galaxies. This is taken as evidence againstthe traditional view that all elliptical galaxies formed early and now evolveonly passively. The frequency of such star forming elliptical galaxies is a fewtenths of a percent in the sample, but increases to 3% if we include active S0galaxies. We may identify these galaxies as probable progenitors of so-calledE+A galaxies that show the strong Balmer absorption feature of A starssuperimposed on an old star population. The approximate match of the abundanceof active elliptical plus S0 galaxies with that of E+A galaxies indicates thatthe 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 galaxypopulation, and if we assume a power law for their number evolution, theabundance of early-type galaxies at $z=1$ is about 30% less than that at $z=0$.Comment: Accepted for publication in Astrophys. J. Letter