Some post-starburst E+A galaxies are not truly post starburst

Dalya Baron, Hagai Netzer, K. Decker French, Dieter Lutz, Ric I. Davies, J. Xavier Prochaska

Published 2022-04-25Version 1

Post-starburst E+A galaxies are believed to be systems in a rapid transition between major merger starbursts and quiescent ellipticals. Their optical spectrum is dominated by A-type stars, suggesting a significant starburst that was quenched recently. While optical observations of post-starburst galaxies suggest little ongoing star formation, they have been shown to host significant molecular gas reservoirs. This led to the suggestion that gas consumption or expulsion are not required to end the starburst, and that star formation is suppressed by turbulent heating of the molecular gas. We present NOEMA continuum and CO(1-0) observations of 15 post-starburst galaxies, and collect CO measurements in post-starburst galaxies from the literature. Using archival far-infrared observations, we show that the majority of these systems host obscured star formation, with some showing far-infrared emission that is comparable to those of luminous and ultraluminous infrared galaxies. Once far-infrared star formation rates are used, these systems show similar SFR-$M_{\mathrm{H_2}}$ and Kennicutt-Schmidt relations to those observed in star-forming and starburst galaxies. In particular, there is no need to hypothesize star formation quenching by processes other than the consumption of molecular gas by star formation. The combination of optical, far-infrared, and CO observations indicates that some regions within these galaxies have been recently quenched, while others are still forming stars at a high rate. We find little connection between the post-burst age of the optically-thin quenched regions and the star formation rate in the obscured regions. All this calls into question the traditional classification of E+A galaxies.