Now You See It, Now You Don’t: Star Formation Truncation Precedes the Loss of Molecular Gas by ∼100 Myr in Massive Poststarburst Galaxies at z ∼ 0.6

Abstract

We use ALMA observations of CO(2–1) in 13 massive (M * ≳ 1011 M ⊙) poststarburst galaxies at z ∼ 0.6 to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The poststarburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (Data Release 14) based on their spectral shapes, as part of the Studying QUenching at Intermediate-z Galaxies: Gas, angu L → ar momentum, and Evolution ( SQuIGG L ⃗ E ) program. Early results showed that two poststarburst galaxies host large H2 reservoirs despite their low inferred star formation rates (SFRs). Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, with M H 2 ≳ 10 9 M ⊙. Given their high stellar masses, this mass limit corresponds to an average gas fraction of 〈 f H 2 ≡ M H 2 / M * 〉 ∼ 7 % or ∼14% using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with the D n 4000 spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support this empirical finding: galaxies that quenched ≲150 Myr prior to observation host detectable CO(2–1) emission, while older poststarburst galaxies are undetected. The large H2 reservoirs and low SFRs in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100–200 Myr, the SQuIGG L ⃗ E galaxies require the additional and efficient heating or removal of cold gas to bring their low SFRs in line with standard H2 scaling relations.

Star Formation Suppression by Tidal Removal of Cold Molecular Gas from an Intermediate-redshift Massive Post-starburst Galaxy

“…Sample galaxies show no radial age gradients, implying quenching took place uniformly throughout the galaxies (Setton et al 2020). Previous ALMA observations of a sample of SQuIGG LE post-starbursts found that the quenching of star formation surprisingly does not require the total removal/destruction of the cold molecular gas reservoir, but this cold gas does rapidly disappear over the ensuing ∼100 Myr post-quenching (Suess et al 2017;Bezanson et al 2022).…”

The Compact Structures of Massive z ∼ 0.7 Post-starburst Galaxies in the SQuIGGL⃗E Sample

“…However, in a previous study of CO(2-1) for a small (13 galaxies) subsample of  L SQuIGG E, we do not see evidence of centrally concentrated molecular gas or continuum emission in the detected galaxies at ∼1″ resolution. We measure molecular gas effective radii on the order of kiloparsecs and do not detect continuum emission in all but one galaxy (Bezanson et al 2022). As such, we assume dust obscuration informed by our best fits to IR spectrophotometric data.…”

“…Crucially, these spectra allow us to characterize the properties of the burst to measure the time since star formation was quenched and to track the evolution of the sample in key properties as a function of the time since quenching. This sample has allowed us to study in detail the link between the star formation shutdown and active galactic nucleus (AGN) incidence (Greene et al 2020), molecular gas content (Suess et al 2017;Bezanson et al 2022), and the incidence of mergers (M. Verrico et al 2022, in preparation).…”

Recovering the Star Formation Histories of Recently Quenched Galaxies: The Impact of Model and Prior Choices

“…Quantifying how long galaxies have been quenched allows us to construct a timeline of how various galaxy properties evolve after star formation shuts down. Because it is impossible to watch a single galaxy evolve through the quenching process, cross-sectional studies using accurate post-quenching ages are the only way to gain an understanding of how galaxy structure, active galactic nuclei (AGN) activity, molecular gas contents, and other key properties change throughout the quenching process (e.g., French et al 2018;Bezanson et al 2022).…”