THE CURRENT STAR FORMATION RATE OF K+A GALAXIES

We derive the stacked 1.4 GHz flux from the FIRST survey for 811 K+A galaxies selected from the Sloan Digital Sky Survey Data Release 7. For these objects we find a mean flux density of 56 ± 9 μJy. A similar stack of radio-quiet white dwarfs yields an upper limit of 43 μJy at a 5σ significance to the flux in blank regions of the sky. This implies an average star formation rate of 1.6 ± 0.3 M☉ yr–1 for K+A galaxies. However, the majority of the signal comes from ∼4% of K+A fields that have aperture fluxes above the 5σ 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☉ yr–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 active galactic nuclei being present.