The current star formation rate of K+A galaxies

Abstract

We derive the stacked 1.4 GHz flux from FIRST (Faint Images of the Radio Sky at Twenty Centimeters) survey for 811 K+A galaxies selected from the SDSS DR7. For these objects we find a mean flux density of $56\pm 9$ $\mu$Jy. A similar stack of radio-quiet white dwarfs yields an upper limit of 43 $\mu$Jy at a 5$\sigma$ significance to the flux in blank regions of the sky. This implies an average star formation rate of 1.6 $\pm$ 0.3 M$_{\odot}$ year$^{-1}$ for K+A galaxies. However the majority of the signal comes from $\sim$4% of K+A fields that have aperture fluxes above the $5\sigma$ noise level of the FIRST survey. A stack of the remaining galaxies shows little residual flux consistent with an upper limit on star formation of 1.3 M$_{\odot}$ year$^{-1}$. Even for a subset of 456 `young' (spectral ages $<$ 250 Myr) K+A galaxies we find that the stacked 1.4 GHz flux is consistent with no current star formation. Our data suggest that the original starburst has been terminated in the majority of K+A galaxies, but that this may represent part of a duty cycle where a fraction of these galaxies may be active at a given moment with dusty starbursts and AGNs being present.