Selection and photometric properties of K+A galaxies

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<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 H alpha 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 H alpha EW. These K+A (or E+A) galaxies must have dramatically decreased their star formation rates over the last similar to 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 similar to 10(-4) h(3) Mpc(-3) Gyr(-1) at redshift z similar to 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.