Danielle Nielsen, Roberto De Propris, Susan E. Ridgway, Tomotsugu Goto
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.
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http://arxiv.org/abs/1106.0757
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