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

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 dustto-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 [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 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.

The Evolution of the Interstellar Medium in Post-starburst Galaxies

“…As a result, 37 PSBs from French et al (2018a), 12 PSBs from Alatalo et al (2016a), and 11 PSBs from Rowlands et al (2015) have detections in 3 Herschel bands 2 , constituting our final sample of 58 PSBs in total 3 . The 37 PSBs from French et al (2018a) are labeled EAH01-EAH18 and EAS01-EAS15, consistent with the nomenclature in French et al (2015) and Smercina et al (2018), and F34-F37, for those without previous names. Their redshifts are 0.02 < z < 0.11.…”

“…As previous studies have shown that FIR (λ 40µm) photometry is crucial (da Cunha et al 2008;Dale et al 2012), we define our sample as those galaxies among the aforemen-tioned three samples of PSBs with archival FIR data. In addition to utilizing the processed WISE and Herschel data from Smercina et al (2018) for 33 PSBs from French et al (2015), we searched for Herschel observations of other galaxies in these three samples in the PACS Point Source Catalog ) and SPIRE Point Source Catalog (Schulz et al 2017), available at the NASA/IPAC Infrared Science Archive (IRSA) 1 . As a result, 37 PSBs from French et al (2018a), 12 PSBs from Alatalo et al (2016a), and 11 PSBs from Rowlands et al (2015) have detections in 3 Herschel bands 2 , constituting our final sample of 58 PSBs in total 3 .…”

“…We establish UV to FIR SEDs for our PSB sample. We incorporate the processed WISE and Herschel data from Smercina et al (2018) for 33 PSBs from French et al (2015) and compile other data from different catalogs. In addition to compiling the Herschel data from PACS/SPIRE Point Source Catalogs, we utilize the archival photometric data from GALEX, SDSS, 2MASS, and WISE.…”

“…Archival data are now available for a statistically significant PSB sample, making it possible to calibrate the relation between M dust and CO-traced M H2 for the first time, as well as to examine the evolution of M dust over a wide range of post-burst ages. Smercina et al (2018) derived M dust for 33 PSBs with CO detections from French et al (2015) and investigated the evolution of their ISM properties in detail. In contrast, here we search all available archival IR data for three large PSB samples from French et al (2018a), Alatalo et al (2016a), and Rowlands et al (2015) and derive M dust for those 58 PSBs with sufficient IR data.…”

The Colors and Sizes of Recently Quenched Galaxies: A Result of Compact Starburst before Quenching

“…dependence on SFR. While the shapes of attenuation curves can vary (Kong et al 2004;Kriek & Conroy 2013;Salim et al 2018) and the dust property can be different in the burst component (Smercina et al 2018), exploring its effect on structures over the entire parameter space is beyond the scope of this paper. Instead, we show an illustrative case on how spatially non-uniform attenuation may affect the color and size.…”