After The Fall: The Dust and Gas in E+A Post-Starburst Galaxies

TL;DR

This study investigates the ISM properties of E+A post-starburst galaxies, revealing significant gas and dust presence, unique infrared features, and a substantial decline in star formation activity, challenging previous notions of their gas-poor nature.

Contribution

It provides the first comprehensive infrared spectroscopic survey of E+A galaxies, showing they retain substantial ISM and exhibit distinctive spectral features, indicating ongoing ISM heating and complex star formation cessation.

Findings

High gas and dust masses in post-starbursts exceed stellar recycling expectations.

Unique IR spectral features with dominant PAH emission and H₂ rotational lines.

Star formation has declined by over two orders of magnitude, but ISM is not fully expelled.

Abstract

The traditional picture of post-starburst galaxies as dust- and gas-poor merger remnants, rapidly transitioning to quiescence, has been recently challenged. Unexpected detections of a significant ISM in many post-starbursts raise important questions. Are they truly quiescent and, if so, what mechanisms inhibit further star formation? What processes dominate their ISM energetics? We present an infrared spectroscopic and photometric survey of 33 SDSS-selected E+A post-starbursts, aimed at resolving these questions. We find compact, warm dust reservoirs with high PAH abundances, and total gas and dust masses significantly higher than expected from stellar recycling alone. Both PAH/TIR and dust-to-burst stellar mass ratios are seen to decrease with post-burst age, indicative of the accumulating effects of dust destruction and an incipient transition to hot, early-type ISM properties. Their infrared spectral properties are unique, with dominant PAH emission, very weak nebular lines, unusually strong H$_{2}$ rotational emission, and deep ${\rm [C\, II]}$ deficits. There is substantial scatter among SFR indicators, and both PAH and TIR luminosities provide overestimates. Even as potential upper limits, all tracers show that the SFR has typically experienced a more than two order-of-magnitude decline since the starburst, and that the SFR is considerably lower than expected given both their stellar masses and molecular gas densities. These results paint a coherent picture of systems in which star formation was, indeed, rapidly truncated, but in which the ISM was $\textit{not}$ completely expelled, and is instead supported against collapse by latent or continued injection of turbulent or mechanical heating. The resulting aging burst populations provide a "high-soft" radiation field which seemingly dominates the E+As' unusual ISM energetics.