The current star formation rate of K+A galaxies

TL;DR

This study uses radio observations to estimate the current star formation rates in K+A galaxies, finding most have little ongoing star formation, indicating recent starburst termination, with some showing signs of active star formation or AGN activity.

Contribution

It provides the first statistical radio-based assessment of star formation activity in K+A galaxies, revealing most have ceased star formation, with a subset showing ongoing activity or AGN presence.

Findings

Average star formation rate in K+A galaxies is about 1.6 M$_{\

The majority of K+A galaxies show little to no current star formation.

A small fraction (~4%) of K+A fields exhibit significant radio flux indicating active star formation or AGN.

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.