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1. Molecular Gas Excitation in z ~ 0.7 Gas-rich Poststarburst Galaxies from SQuIGGLE

   https://doi.org/10.3847/1538-4357/ae64e7  

   D’Onofrio, Vincenzo R; Spilker, Justin S; Bezanson, Rachel; Feldmann, Robert; Goulding, Andy D; Greene, Jenny E; Kriek, Mariska; Kumar, Anika; Luo, Yuanze; Narayanan, Desika; et al (May 2026, The Astrophysical Journal)

   Many poststarburst galaxies at z ∼ 0.7 have been shown to retain substantial molecular gas reservoirs yet host low ongoing star formation, suggesting that the remaining gas may be inefficient at forming stars during the early postburst phase. We present new Atacama Large Millimeter/submillimeter Array CO(5-4) observations of nine gas-rich poststarburst galaxies at z ~ 0.7 from the Studying Quenching in Intermediate-z Galaxies: Gas, anguLar momentum, and Evolution (SQuIGGLE) survey, providing a view of the molecular gas excitation in these systems. Combined with existing CO(2-1) data, we detect CO(5─4) in eight out of nine targets and find that most have moderate CO excitation with r52 = L'CO(5−4)/L'CO(2−1) ~ 0.1−0.3 . These systems show no clear trend between r52 and either total or surface-density of star formation. Specifically, all objects have ΣSFR ∼ 0.01─1 M⊙ yr−1 kpc−2, consistent with compact, modest star formation, even when allowing for buried activity, as these galaxies decline from their peak. One object, J1448+1010, which has clear optical, mid-infrared, and radio indicators of an active galactic nucleus, is an outlier with r52 ~ 0.6; its elevated excitation likely requires significant nonstellar heating, with a contribution from potentially obscured star formation. Together, most gas-rich SQuIGGLE poststarbursts have moderately excited molecular gas alongside little to modest star-forming activity, indicating that the remaining gas hosts relatively suppressed star formation efficiencies instead of strong buried starburst activity. 

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2. Cold Gas in a Post-starburst Pair in the z ~ 1.4 HeavyMetal Survey: Major Mergers as a Pathway to Retain Gas in Quenched Galaxies

   https://doi.org/10.3847/1538-4357/ae0933  

   Suess, Katherine A; Beverage, Aliza G; Kriek, Mariska; Spilker, Justin S; Bezanson, Rachel; D’Onofrio, Vincenzo R; Greene, Jenny E; Lin, Jamie; Luo, Yuanze; Narayanan, Desika; et al (October 2025, The Astrophysical Journal)

   Recent observations at low redshift have revealed that some post-starburst galaxies retain significant molecular gas reservoirs despite low ongoing star formation rates, challenging most theoretical predictions that rely on heating or expelling gas reservoirs to shut down star formation. To test whether this finding holds during the peak epoch of quenching, here we present Atacama Large Millimeter/submillimeter Array CO(2–1) observations of five spectroscopically confirmed massive post-starburst galaxies at z~ 1.4 from the HeavyMetal survey. While four galaxies are undetected in CO emission, we detect MH2 ∼ 10^9.7 M⊙ of molecular gas in one system. The detected system is a close pair of two massive (M_star = 10^11.1−11.2 M⊙) post-starburst galaxies with no clear tidal features, likely caught in the early stages of a major merger. These results suggest that mergers may be a key factor in retaining molecular gas while simultaneously suppressing star formation in quenched galaxies at high redshift, possibly by driving increased turbulence that decreases star formation efficiency. Our nondetected galaxies have postburst ages >200 Myr, consistent with results at z < 1; however, our gas-rich post-starburst pair is significantly older than typical gas-rich quenched systems at low redshift. Our results highlight the importance of major mergers in shaping the cold gas content of quiescent galaxies during the peak epoch of quenching. 

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

   https://doi.org/10.3847/2041-8213/ac75ea  

   Spilker, Justin S.; Suess, Katherine A.; Setton, David J.; Bezanson, Rachel; Feldmann, Robert; Greene, Jenny E.; Kriek, Mariska; Lower, Sidney; Narayanan, Desika; Verrico, Margaret (August 2022, The Astrophysical Journal Letters)

   Abstract Observations and simulations have demonstrated that star formation in galaxies must be actively suppressed to prevent the formation of overly massive galaxies. Galactic outflows driven by stellar feedback or supermassive black hole accretion are often invoked to regulate the amount of cold molecular gas available for future star formation but may not be the only relevant quenching processes in all galaxies. We present the discovery of vast molecular tidal features extending up to 64 kpc outside of a massive z = 0.646 post-starburst galaxy that recently concluded its primary star-forming episode. The tidal tails contain (1.2 ± 0.1) × 10 10 M ⊙ of molecular gas, 47% ± 5% of the total cold gas reservoir of the system. Both the scale and magnitude of the molecular tidal features are unprecedented compared to all known nearby or high-redshift merging systems. We infer that the cold gas was stripped from the host galaxies during the merger, which is most likely responsible for triggering the initial burst phase and the subsequent suppression of star formation. While only a single example, this result shows that galaxy mergers can regulate the cold gas contents in distant galaxies by directly removing a large fraction of the molecular gas fuel, and plausibly suppress star formation directly, a qualitatively different physical mechanism than feedback-driven outflows. 

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4. Massive Quenched Galaxies at z ˜ 0.7 Retain Large Molecular Gas Reservoirs

   Suess, Katherine A.; Bezanson, Rachel; Spilker, Justin S.; Kriek, Mariska; Greene, Jenny E.; Feldmann, Robert; Hunt, Qiana; Narayanan, Desika (January 2017, APJ)

   The physical mechanisms that quench star formation, turning blue star-forming galaxies into red quiescent galaxies, remain unclear. In this Letter, we investigate the role of gas supply in suppressing star formation by studying the molecular gas content of post-starburst galaxies. Leveraging the wide area of the Sloan Digital Sky Survey, we identify a sample of massive intermediate-redshift galaxies that have just ended their primary epoch of star formation. We present Atacama Large Millimeter/submillimeter Array CO(2-1) observations of two of these post-starburst galaxies at z ˜ 0.7 with {M}* ˜ 2× {10}11 {M}⊙ . Their molecular gas reservoirs of (6.4+/- 0.8) × {10}9 {M}⊙ and (34.0+/- 1.6)× {10}9 {M}⊙ are an order of magnitude larger than comparable-mass galaxies in the local universe. Our observations suggest that quenching does not require the total removal or depletion of molecular gas, as many quenching models suggest. However, further observations are required both to determine if these apparently quiescent objects host highly obscured star formation and to investigate the intrinsic variation in the molecular gas properties of post-starburst galaxies. 

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5. SQuIGGL⃗E : Studying Quenching in Intermediate-z Galaxies—Gas, AnguL⃗ar Momentum, and Evolution

   https://doi.org/10.3847/1538-4357/ac404a  

   Suess, Katherine A.; Kriek, Mariska; Bezanson, Rachel; Greene, Jenny E.; Setton, David; Spilker, Justin S.; Feldmann, Robert; Goulding, Andy D.; Johnson, Benjamin D.; Leja, Joel; et al (February 2022, The Astrophysical Journal)

   Abstract We describe the Studying Quenching in Intermediate- z Galaxies: Gas, angu L → ar momentum, and Evolution ( SQuIGG L ⃗ E ) survey of intermediate-redshift post-starburst galaxies. We leverage the large sky coverage of the Sloan Digital Sky Survey to select ∼ 1300 recently quenched galaxies at 0.5 < z ≤ 0.9 based on their unique spectral shapes. These bright, intermediate-redshift galaxies are ideal laboratories to study the physics responsible for the rapid quenching of star formation: they are distant enough to be useful analogs for high-redshift quenching galaxies, but low enough redshift that multiwavelength follow-up observations are feasible with modest telescope investments. We use the Prospector code to infer the stellar population properties and nonparametric star formation histories (SFHs) of all galaxies in the sample. We find that SQuIGG L ⃗ E galaxies are both very massive ( M * ∼ 10 11.25 M ⊙ ) and quenched, with inferred star formation rates ≲1 M ⊙ yr −1 , more than an order of magnitude below the star-forming main sequence. The best-fit SFHs confirm that these galaxies recently quenched a major burst of star formation: >75% of SQuIGG L ⃗ E galaxies formed at least a quarter of their total stellar mass in the recent burst, which ended just ∼200 Myr before observation. We find that SQuIGG L ⃗ E galaxies are on average younger and more burst-dominated than most other z ≲ 1 post-starburst galaxy samples. This large sample of bright post-starburst galaxies at intermediate redshift opens a wide range of studies into the quenching process. In particular, the full SQuIGG L ⃗ E survey will investigate the molecular gas reservoirs, morphologies, kinematics, resolved stellar populations, active galactic nucleus incidence, and infrared properties of this unique sample of galaxies in order to place definitive constraints on the quenching process. 

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