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arxiv: 2606.26320 · v1 · pith:YVHPQHZHnew · submitted 2026-06-24 · 🌌 astro-ph.GA

Probing the molecular gas content of galaxies in an over-dense group at z~0.7: a test case for environmental quenching

Jonathan Freundlich , Beno\^it Epinat , Thierry Contini , Philippe Salom\'e , Fran\c{c}oise Combes , Baptiste Jego , Davor Krajnovi\'c , Wilfried Mercier
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Constanza Mu\~noz L\'opez Matthieu B\'ethermin Leindert Boogaard Rodrigo Herrera-Camus Diana Ismail Fangzhou Jiang Katarina Kraljic Florent Renaud Sandro Tacchella
This is my paper

Pith reviewed 2026-06-26 01:17 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords molecular gasgalaxy groupsenvironmental quenchingCO emissionz~0.7gas depletionCOSMOS fieldstar formation
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The pith

Galaxies in a dense group at z~0.7 show molecular gas contents reduced by 0.5 dex relative to field scaling relations.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper observes CO(2-1) emission from galaxies in the COSMOS-Gr30 group to test whether dense environments at intermediate redshift reduce molecular gas reservoirs. Individual detections occur in four members, with upper limits for the rest, and the stack reveals average gas masses 0.5 dex below expectations from field relations. This corresponds to gas fractions only 20-40 percent of those in typical main-sequence galaxies of matching mass and star-formation rate. An additional upper limit shows that the extended ionized gas structure contains less than 2 times 10^10 solar masses of molecular gas. These measurements indicate that environmental mechanisms in the group are removing or suppressing cold gas.

Core claim

Stacked CO(2-1) measurements of the group members yield molecular gas contents lower by approximately 0.5 dex than predicted by literature field scaling relations, implying gas fractions 20 to 40 percent of those in main-sequence galaxies; the 30 m data additionally constrain the molecular gas mass associated with the large ionized structure to less than 2 times 10^10 solar masses.

What carries the argument

Comparison of stacked CO-derived molecular gas masses against field scaling relations for galaxies of matched stellar mass and star-formation rate.

If this is right

  • Environmental processes in this group are removing or suppressing molecular gas on short timescales.
  • The group environment captures quenching in action at z approximately 0.7.
  • Less than one-third of the gas in the intra-group medium exists in a cold, star-forming phase.
  • The reduction applies on average to the full group population, including undetected members.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Repeating the same stacking approach on additional groups at similar redshift would test whether the depletion is common or specific to this filament intersection.
  • The result supplies a concrete target for simulations that track cold-gas stripping or starvation inside groups.
  • If the ionized structure traces ongoing stripping, deeper CO maps could reveal whether the missing molecular gas has already been heated or dispersed.

Load-bearing premise

Literature field scaling relations give an unbiased prediction of molecular gas content for galaxies of the same stellar mass and star-formation rate outside groups.

What would settle it

A re-analysis or new observation showing that the stacked CO flux matches field expectations once selection and stacking biases are fully accounted for would falsify the reported reduction.

Figures

Figures reproduced from arXiv: 2606.26320 by Baptiste Jego, Beno\^it Epinat, Constanza Mu\~noz L\'opez, Davor Krajnovi\'c, Diana Ismail, Fangzhou Jiang, Florent Renaud, Fran\c{c}oise Combes, Jonathan Freundlich, Katarina Kraljic, Leindert Boogaard, Matthieu B\'ethermin, Philippe Salom\'e, Rodrigo Herrera-Camus, Sandro Tacchella, Thierry Contini, Wilfried Mercier.

Figure 1
Figure 1. Figure 1: JWST image (F277W band, logarithmic scale, arbitrary [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Position of the 17 galaxies of COSMOS-Gr30 within the [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: NOEMA CO(2-1) spectra of the four detected galaxies in the group CGr30, namely #71, #105, #119, and #131. The S [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: Observed relation between the SFR and the intrinsic [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 4
Figure 4. Figure 4: Stacked NOEMA CO(2-1) spectra over all galaxies in [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Molecular gas mass divided by that expected from the [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: CO(2-1) WILMA spectrum obtained with the 30m tele [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
read the original abstract

To probe the impact of group environment on molecular gas reservoirs at intermediate redshift, we observed the CO(2-1) emission in the galaxy group COSMOS-Gr30 at $z \sim 0.7$ with IRAM's NOEMA and 30m telescopes. This dense environment, located at the intersection of large-scale cosmic web filaments, has the specificity to host a large ($\sim 10^{4}$ kpc$^{2}$) ionized gas structure revealed by MUSE. We detect CO emission in four galaxies of the group at $\mathrm{S/N} > 5$ and derive upper limits for the remaining group members with secure spectroscopic redshifts. Stacked measurements indicate that group galaxies exhibit on average molecular gas contents reduced by $\sim 0.5$ dex relative to field scaling relations, corresponding to gas fractions that are $20\%$ to $40\%$ of those found in typical main-sequence galaxies. Although the uncertainties are significant, this suggests that environmental processes efficiently deplete molecular gas reservoirs in the galaxies of this group. The 30m observations place an upper limit on the molecular gas associated with the extended ionized structure, $M_{\rm gas} < 2 \times 10^{10} \rm M_\odot$, implying that less than a third of the gas in the intra-group medium is in a cold, star-forming phase. Together, these results contribute to show how environmental mechanisms in dense group environments act to remove or suppress molecular gas within galaxies, capturing quenching processes in action.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper reports NOEMA and IRAM 30m CO(2-1) observations of the COSMOS-Gr30 galaxy group at z~0.7. Four group members are detected at S/N>5, with upper limits for the remainder; stacking yields an average molecular gas content ~0.5 dex below field scaling relations (gas fractions 20-40% of main-sequence values), interpreted as evidence for environmental depletion. An upper limit M_gas < 2e10 M_sun is placed on the extended ionized structure seen by MUSE, implying <1/3 of intra-group gas is in a cold phase.

Significance. If the offset holds after accounting for baselines and systematics, the work supplies a useful test case of environmental effects on molecular gas at intermediate redshift in a filament-intersection group. The multi-telescope approach and constraint on the intra-group medium are strengths. The small detection count and external-relation comparison keep the quantitative claim provisional, consistent with the abstract's own caveats on uncertainties.

major comments (2)
  1. [Abstract] Abstract and results section: the headline 0.5 dex deficit (and 20-40% gas-fraction claim) is derived from a stack of four detections plus upper limits; without an explicit table or subsection comparing the group's M*–SFR distribution to the exact literature calibration samples used for the field relations, it is impossible to rule out a selection-driven offset in the baseline.
  2. [Methods] Methods/results: the stacking weights, handling of non-detections, and adopted CO-to-H2 conversion factor (listed as a free parameter in the axiom ledger) are not specified with sufficient quantitative detail to demonstrate that the procedure does not systematically underestimate the mean; this is load-bearing for the reported environmental depletion.
minor comments (2)
  1. [Abstract] Abstract: the statement that uncertainties are 'significant' should be accompanied by the actual error on the 0.5 dex offset and on the gas-fraction range.
  2. [Results] The 30 m upper limit on the extended structure is a useful ancillary result but would benefit from a short dedicated paragraph clarifying the assumed line width and excitation.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive report and the recommendation for major revision. We address the two major comments below by expanding the manuscript with the requested comparisons and quantitative details on the analysis. These changes will strengthen the presentation without altering the core results or conclusions.

read point-by-point responses

  1. Referee: [Abstract] Abstract and results section: the headline 0.5 dex deficit (and 20-40% gas-fraction claim) is derived from a stack of four detections plus upper limits; without an explicit table or subsection comparing the group's M*–SFR distribution to the exact literature calibration samples used for the field relations, it is impossible to rule out a selection-driven offset in the baseline.

    Authors: We agree that an explicit side-by-side comparison of the M*–SFR distribution is necessary to address potential selection effects. In the revised manuscript we will add a dedicated subsection (in Results) and an accompanying table that tabulates the stellar masses, SFRs, and sSFRs for all COSMOS-Gr30 members with secure redshifts, together with the median values and ranges from the precise field samples used to construct the scaling relations (e.g., the Tacconi et al. or Genzel et al. compilations referenced in the text). This will allow direct assessment of whether the group galaxies occupy the same locus as the field calibration set. revision: yes

  2. Referee: [Methods] Methods/results: the stacking weights, handling of non-detections, and adopted CO-to-H2 conversion factor (listed as a free parameter in the axiom ledger) are not specified with sufficient quantitative detail to demonstrate that the procedure does not systematically underestimate the mean; this is load-bearing for the reported environmental depletion.

    Authors: We acknowledge that the current description of the stacking procedure lacks the quantitative detail required. The revised Methods section will explicitly state: (i) the weighting scheme applied to the four detections and upper limits (inverse-variance weighting with the precise formula), (ii) the statistical treatment of non-detections (including whether a Kaplan–Meier estimator, survival analysis, or conservative upper-limit substitution was used), and (iii) the adopted CO-to-H2 conversion factor together with its justification, the range explored, and how its uncertainty is propagated into the final 0.5 dex offset. These additions will demonstrate that the mean is not systematically underestimated. revision: yes

Circularity Check

0 steps flagged

No significant circularity; comparison relies on external literature scaling relations

full rationale

The paper's central result is a direct observational comparison: stacked CO-derived molecular gas masses for the group galaxies are reported ~0.5 dex below published field scaling relations for M_H2 vs. M* and SFR. No internal equations, fitted parameters, or self-citations are used to define or force this offset; the deficit is measured against independent external calibrations. The stacking procedure and upper-limit handling are described as standard and do not reduce the reported gas fractions to a quantity defined inside the paper. This is the most common honest non-finding for observational papers that benchmark against literature relations.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Molecular-gas masses are derived using standard CO-to-H2 conversion assumptions and compared against external field relations; no new physical entities are introduced.

free parameters (1)
  • CO-to-H2 conversion factor
    Standard factor required to translate observed CO luminosity into molecular hydrogen mass; value taken from literature calibrations.
axioms (1)
  • domain assumption Field scaling relations for molecular gas content versus stellar mass and SFR provide an unbiased reference population.
    Used to define the 0.5 dex reduction and 20-40% gas-fraction range.

pith-pipeline@v0.9.1-grok · 5907 in / 1371 out tokens · 45354 ms · 2026-06-26T01:17:05.997686+00:00 · methodology

discussion (0)

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