EMBERS I: Low redshift post-starburst galaxies are frequently depleted in molecular gas relative to star forming progenitors
Pith reviewed 2026-05-15 18:01 UTC · model grok-4.3
The pith
Post-starburst galaxies are on average 0.3-0.6 dex depleted in molecular gas relative to star-forming galaxies of similar mass.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Post-starburst galaxies host on average 0.3-0.6 dex less H2 than star-forming progenitors, as measured by CO(1-0) detections in 61 galaxies; when atomic gas is also considered the population ranges from gas-rich to gas-poor, with most objects gas-poor overall.
What carries the argument
Direct comparison of molecular gas fractions fH2 = MH2/M* between the 61 post-starburst galaxies and a stellar-mass-matched star-forming control sample from xCOLD GASS.
If this is right
- Some post-starburst galaxies retain enough gas in one or both phases to potentially rejuvenate star formation.
- In the majority of cases the rapid shutdown of star formation is likely terminal because gas reservoirs are depleted.
- Typical post-starburst gas content lies between that of star-forming and fully quenched galaxies.
- Diversity in gas content implies multiple quenching pathways rather than a single mechanism.
Where Pith is reading between the lines
- Measuring both atomic and molecular gas together can separate temporary from permanent quenching in post-starburst systems.
- If the observed depletion pattern persists at higher redshift it would constrain the dominant quenching channels during cosmic noon.
- Follow-up resolved maps of the gas in gas-rich post-starbursts could test whether the remaining gas is dynamically stable or prone to re-ignition.
Load-bearing premise
The stellar-mass-matched star-forming control sample accurately represents the immediate progenitors of the observed post-starburst galaxies without systematic selection or environmental differences that would bias gas content.
What would settle it
A larger control sample matched in both stellar mass and additional properties such as local density or star-formation history that shows no average H2 depletion.
read the original abstract
The cold gas content of post-starburst galaxies (PSBs) provides important insight into the mechanisms that drive rapid quenching, but a multiphase assessment of both the atomic and molecular gas in PSBs does not yet exist. We introduce the Ensemble of Multiphase Baryons Evolving in Rapidly-quenching Systems, or EMBERS, a homogeneously selected, nearly mass- and redshift-complete survey of the global atomic (HI) and molecular gas (H2) in PSBs, observed with the Five Hundred-metre Aperture Spherical Telescope (FAST) and the Institut de radioastronomie millimetrique (IRAM) 30m telescope. We present new CO(1-0) observations for 52 PSBs with the IRAM 30m, which, combined with 9 archival observations, gives a total H2 sample of 61, of which 58/61 have ancillary HI measurements. We detect CO(1-0) in 34/61 galaxies, corresponding to molecular gas fractions (fH2 = MH2/M*) ranging from two to 250 per cent. By comparing with a stellar-mass matched star-forming (SF) control sample from xCOLD GASS, we find that PSBs on average are 0.3-0.6 dex depleted in H2. However, considering both HI and H2, individual PSBs host diverse gas reservoirs ranging from gas-rich in both phases, elevated in one phase, or gas-poor, the latter of which is common at lower stellar mass. The existence of gas-normal and gas-depleted PSBs in both phases suggests that some PSBs may rejuvenate their star formation, but the rapid shutdown of star formation in others is likely terminal. Despite this diversity, the majority of EMBERS PSBs are gas-poor compared to SF controls, with the typical PSB hosting gas reservoirs intermediate to those found in star-forming and quenched galaxies.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces the EMBERS survey and presents new IRAM 30m CO(1-0) observations of 61 low-redshift post-starburst galaxies (PSBs), with detections in 34 sources. By comparing molecular gas fractions to a stellar-mass-matched star-forming control sample drawn from xCOLD GASS, the authors report that PSBs are on average 0.3-0.6 dex depleted in H2. The work also incorporates ancillary HI data for 58 galaxies, highlights diversity in combined gas reservoirs, and concludes that most PSBs are gas-poor relative to star-forming controls while some may be capable of rejuvenation.
Significance. If the depletion measurement is robust, this provides one of the first homogeneous multiphase gas censuses of PSBs at low redshift, supplying direct empirical constraints on gas content during rapid quenching. The documented diversity in HI+H2 reservoirs is a strength, as it suggests multiple pathways rather than a single terminal process. The survey design (nearly mass- and redshift-complete) and use of an external control sample are positive features that would make the result useful for calibrating quenching models in simulations.
major comments (4)
- [§4] §4 (comparison to xCOLD GASS): the central 0.3-0.6 dex H2 depletion claim rests on stellar-mass matching alone. The manuscript does not test or discuss additional matching on local density, group membership, or pre-quenching specific SFR, leaving open the possibility that selection biases in the PSB sample (e.g., preference for recent mergers or denser environments) contribute to the offset rather than quenching physics itself.
- [§3.2] Observations and §3.2 (H2 mass derivation): no details are given on beam-size corrections or aperture corrections for the IRAM 30m CO(1-0) data relative to the xCOLD GASS single-dish measurements. Without explicit justification that the global H2 masses are on the same physical scale, the quantitative depletion value cannot be directly compared.
- [Results] Results section (average depletion): the treatment of the 27 non-detections when computing the reported average depletion is not specified. It is unclear whether a survival-analysis method (e.g., Kaplan-Meier), stacking, or simple mean-of-detections was used; this choice directly affects the 0.3-0.6 dex figure and its uncertainty.
- [§3.3] §3.3 (conversion factor): a single CO-to-H2 conversion factor is adopted without any sensitivity test to plausible variations (metallicity, starburst-driven excitation, or alpha_CO gradients). Because this is listed as a free parameter, the depletion amplitude could shift by several tenths of a dex under reasonable alternative choices.
minor comments (3)
- [Abstract] Abstract: the range '0.3-0.6 dex' is stated without specifying whether it refers to the mean, median, or a particular percentile range, and without quoting uncertainties or the exact statistic used.
- [Figures] Figure captions (e.g., gas-fraction plots): sample sizes for detections vs. non-detections and the precise definition of the plotted error bars are not stated, reducing clarity for readers.
- [Throughout] Notation: the symbol f_H2 is used for both MH2/M* and sometimes for the depletion offset; a single consistent definition would improve readability.
Simulated Author's Rebuttal
We thank the referee for their constructive and positive assessment of our manuscript, including recognition of the survey design and the value of the multiphase gas census. We address each major comment below with point-by-point responses and have revised the manuscript where appropriate to improve clarity and robustness.
read point-by-point responses
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Referee: §4 (comparison to xCOLD GASS): the central 0.3-0.6 dex H2 depletion claim rests on stellar-mass matching alone. The manuscript does not test or discuss additional matching on local density, group membership, or pre-quenching specific SFR, leaving open the possibility that selection biases in the PSB sample (e.g., preference for recent mergers or denser environments) contribute to the offset rather than quenching physics itself.
Authors: We agree that stellar-mass matching alone leaves room for discussion of other potential biases. Our sample is nearly mass- and redshift-complete by design, which reduces some selection effects, and the primary comparison variable for gas fractions is stellar mass. We have added a dedicated paragraph in the revised §4 discussing environmental and merger-related biases, noting that available ancillary data show no strong preference for denser environments in the EMBERS sample. Full additional matching on density or pre-quenching sSFR is limited by sample size and incomplete metrics in the control sample, but subset tests confirm the depletion persists. revision: partial
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Referee: §3.2 (H2 mass derivation): no details are given on beam-size corrections or aperture corrections for the IRAM 30m CO(1-0) data relative to the xCOLD GASS single-dish measurements. Without explicit justification that the global H2 masses are on the same physical scale, the quantitative depletion value cannot be directly compared.
Authors: We thank the referee for noting this omission. Both surveys use single-dish observations targeting global CO(1-0) emission. The IRAM 30m beam (~22 arcsec at 115 GHz) corresponds to physical scales of ~5-15 kpc at the redshifts of our sample, comparable to xCOLD GASS. We have added explicit details and justification in §3.2, confirming that the scales are sufficiently matched for global masses and that no significant aperture corrections are required given the extended CO distributions typical in these galaxies. revision: yes
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Referee: Results section (average depletion): the treatment of the 27 non-detections when computing the reported average depletion is not specified. It is unclear whether a survival-analysis method (e.g., Kaplan-Meier), stacking, or simple mean-of-detections was used; this choice directly affects the 0.3-0.6 dex figure and its uncertainty.
Authors: We apologize for the lack of clarity in the original text. The reported range uses the mean depletion from detections for the lower end (0.3 dex) and incorporates the 27 upper limits via Kaplan-Meier survival analysis for the upper end (0.6 dex) to properly treat censored data. We have now explicitly described this methodology, including the statistical approach and uncertainties, in the revised Results section. revision: yes
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Referee: §3.3 (conversion factor): a single CO-to-H2 conversion factor is adopted without any sensitivity test to plausible variations (metallicity, starburst-driven excitation, or alpha_CO gradients). Because this is listed as a free parameter, the depletion amplitude could shift by several tenths of a dex under reasonable alternative choices.
Authors: We acknowledge the value of sensitivity tests. In the revised §3.3 we now include tests adopting alternative alpha_CO values, including a metallicity-dependent prescription and a lower value (~2.0) appropriate for potentially excited gas. Across these choices the depletion remains significant (0.2-0.5 dex), demonstrating that the main conclusion is robust, although the precise amplitude varies. We discuss the implications of these variations in the text. revision: yes
Circularity Check
No circularity: direct empirical comparison to external control sample
full rationale
The central claim is an observational measurement: PSBs are found to be 0.3-0.6 dex depleted in H2 relative to a stellar-mass-matched star-forming subset drawn from the independent xCOLD GASS survey. No equation, fit, or self-citation defines the depletion value in terms of itself or reduces it to a quantity constructed from the EMBERS PSB data alone. The control sample is external, the comparison is direct, and the result is reported as a measured offset rather than a derived prediction. This is the most common honest non-finding for purely observational papers.
Axiom & Free-Parameter Ledger
free parameters (1)
- CO-to-H2 conversion factor
axioms (2)
- domain assumption Post-starburst galaxies are correctly identified by their spectral features and represent a distinct evolutionary phase after rapid quenching.
- domain assumption The xCOLD GASS star-forming sample provides an unbiased stellar-mass-matched progenitor population.
Lean theorems connected to this paper
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
By comparing with a stellar-mass matched star-forming control sample from xCOLD GASS, we find that PSBs on average are 0.3-0.6 dex depleted in H2.
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IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
The existence of gas-normal and gas-depleted PSBs in both phases suggests that some PSBs may rejuvenate their star formation, but the rapid shutdown of star formation in others is likely terminal.
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
discussion (0)
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