Abstract
Two different simple measurements of galaxy star formation rate with different timescales are compared empirically on 156,395 fiber spectra of galaxies with r < 17.77 mag taken from the Sloan Digital Sky Survey in the redshift range 0.05 < z < 0.20: a ratio A/K found by fitting a linear sum of an average old stellar population spectrum (K) and average A star spectrum (A) to the galaxy spectrum, and the equivalent width (EW) of the Hα emission line. The two measures are strongly correlated, but there is a small, clearly separated population of outliers from the median correlation that display excess A/K relative to Hα EW. These (or E+A) galaxies must have dramatically decreased their star formation rates over the last ~1 Gyr. The K+A luminosity distribution is similar to that of the total galaxy population. The K+A population appears to be bulge-dominated, but bluer and with higher surface brightness than normal bulge-dominated galaxies; it appears that K+A galaxies will fade with time into normal bulge-dominated galaxies. The inferred rate density for K+A galaxy formation is ~10-4 h3 Mpc-3 Gyr-1 at redshift z ~ 0.1. These events are taking place in the field; the K+A galaxies found in this study do not primarily lie in the high-density environments or clusters typical of bulge-dominated populations.
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