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

arXiv:2606.26320 · astro-ph.GA · Submitted 2026-06-24 · Read on arXiv

Listen

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Probing the molecular gas content of galaxies in an over-dense group at z 0.7".

Jocelyn: To probe how dense group environments affect galaxy evolution,

Vera: First, who's behind it and why it matters.

Title and authors: Vera: So, we're looking at the paper titled "Probing the molecular gas content of galaxies in an over-dense group at z zero point seven: A test case for environmental quenching <ref:2606.26320#pg0,Probing the molecular gas content of galaxies in an over-dense group>." It sounds like they're focusing on a specific, dense environment to see how it affects how much cold gas galaxies have available.

Jocelyn: Yeah, I noticed the title immediately points us toward testing environmental quenching in a group setting at a relatively high redshift of zero point seven <ref:2606.26320#pg0>. It's interesting that they chose such an over-dense environment for this investigation; we always wonder if those conditions are where the most intense processes happen.

Subrahmanyan: From a theoretical standpoint, choosing CGr30 as the test case is smart because its high velocity dispersion means its galaxies are expected to undergo environmental processes within a few gigayears before they merge, which gives us a specific timescale to check against our models of gas depletion.

Vera: Exactly. It’s not just any group; it’s specifically selected because of that high overdensity, and the goal is clearly to see if we can quantify the effects environmental processes have on star formation fuel across different galaxy populations.

Jocelyn: I agree. When you look at the authors, you see a team from various institutions, which suggests they're bringing in different expertise to tackle this complex environmental physics problem.

Subrahmanyan: It’s impressive how they are combining observational data with theoretical expectations to build this test case for quenching mechanisms. That connection between the physical environment and the molecular reservoirs is exactly what we need to study.

The paper's summary: Vera: This paper, "Probing the molecular gas content of galaxies in an over-dense group at z zero point seven: A test case for environmental quenching," essentially sets out to characterize the molecular gas reservoirs within galaxies in this dense group and also look at the diffuse molecular gas surrounding them <ref:2606.26320#pg0,Probing the molecular gas content of galaxies in an over-dense group>.

Jocelyn: It summarizes a few key findings: they found that these group galaxies have on average molecular gas contents that are reduced by about zero point five dex compared to what we see in field galaxies, meaning they only have about twenty to forty percent of the gas you'd expect in a typical main-sequence galaxy.

Subrahmanyan: That reduction is significant because it directly points toward efficient environmental processes depleting those reservoirs, whether through removing the gas entirely or suppressing its ability to turn into stars.

Vera: And they didn't just look at the galaxies; they also investigated potential diffuse molecular gas in the intra-group medium, finding an upper limit on this component suggesting less than a third of that surrounding gas is in a cold, star-forming phase.

Jocelyn: That’s quite a detailed look into both what’s inside and what’s around the galaxies simultaneously; it gives us a consistent picture of how environmental effects are at work in these dense settings.

Subrahmanyan: The paper connects this depletion to various mechanisms, like tidal interactions or ram pressure stripping, suggesting that these processes are capturing quenching processes in action across different scales.

The paper's improvements: Vera: Now, looking at how they improve the study itself, the authors seem to have focused on using complementary tracers to get a more complete picture of the molecular gas content than could be achieved with just one method.

Jocelyn: They used both CO(two-one) emission from NOEMA and observations from the 30m telescope, which allowed them to probe both the galactic reservoirs and the surrounding medium at once, which is a nice way to cross-check their results.

Subrahmanyan: From a modeling perspective, they also compared their derived molecular gas masses against expected values from Tacconi et al. (two thousand twenty) scaling relations, which gives them a baseline to see how far their group galaxies fall relative to typical star-forming galaxies at that redshift and stellar mass <ref:2606.26320#pg0>.

Vera: That comparison is crucial because it shows that the stacked measurements indicate an average trend for CGr30 galaxies having lower molecular gas mass than the typical star-forming galaxies that those scaling relations are based on.

Jocelyn: So, they’re using these scaling relations to show a consistent pattern of suppression in gas content across the entire sample, not just isolated findings for a few objects.

Subrahmanyan: This helps build a stronger case that environmental processes in the group are highly efficient at removing or suppressing the molecular gas required for star formation, which is exactly what we need to constrain our theoretical models.

Conclusion: Vera: So, wrapping up this discussion on "Probing the molecular gas content of galaxies in an over-dense group at z zero point seven: A test case for environmental quenching," it really shows that environmental processes in the CGr30 group are very effective at removing or suppressing the molecular gas needed for star formation, typically down to twenty to forty percent of what we see in normal star-forming galaxies <ref:2606.26320#pg0,Probing the molecular gas content of galaxies in an over-dense group>.

Jocelyn: It’s a pretty comprehensive picture because they linked the depletion within the galaxies with that extended gas structure, suggesting that those processes causing internal depletion are connected to the presence of non-pristine gas structures around them.

Subrahmanyan: My take is that this finding strongly supports the idea that environmental mechanisms in dense group environments act to remove or suppress molecular gas within galaxies, which helps us capture quenching processes in action and informs how we model galaxy evolution over time.

Vera: It’s an important piece of evidence because it solidifies the link between these external interactions and the observed fuel supply for star formation, and I think this paper is a solid foundation for future work.

Jocelyn: I agree; it lays down a clear constraint on what we expect from gas content in these specific environments, which sets a benchmark for what future surveys should aim to measure.

Subrahmanyan: Indeed, constraining the molecular gas mass of galaxies in this CGr30 group provides concrete data that we can use to refine the theoretical understanding of how dense environments drive galaxy evolution.

Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, France · Aix Marseille Univ, CNRS, CNES · French-Chilean Laboratory for Astronomy · Canada-France-Hawaii Telescope · Observatoire de Paris, LUX · Leibniz-Institut für Astrophysik Potsdam (AIP) · Leiden Observatory, Leiden University · Millennium Nucleus for Galaxies

astro-ph.GA

Submitted: 2026-06-24

Updated: 2026-10-01

Comments: 15 pages, 15 figures, accepted by A&A

License: http://creativecommons.org/licenses/by-nc-sa/4.0/

Importance score: 83/100

The gist: To probe how dense group environments affect galaxy evolution, this study observed molecular gas reservoirs in the over-dense group COSMOS-Gr30 at redshift z ∼ 0.7 using IRAM’s NOEMA and 30m

Key concepts

Molecular Gas Reservoirs: This refers to the cold, dense gas clouds within galaxies that contain the raw material necessary for stars to form. The study measured how much of this gas is present in group galaxies compared to normal galaxies, showing it is often reduced by half.
Environmental Quenching: This describes the physical processes in dense environments (like galaxy groups) that stop or severely reduce a galaxy's ability to form new stars. The paper suggests that removing or suppressing the molecular gas supply is a key mechanism for this quenching.
CO(2-1) Emission: This is a specific signal astronomers use to detect and measure the amount of molecular gas in galaxies. By observing this emission using instruments like NOEMA, scientists can calculate the total mass of cold gas present in the group galaxies.
Depletion Time (tdepl): This measures how quickly the molecular gas supply is being used up or removed from a galaxy over time. The study found that group galaxies deplete their gas much faster than typical star-forming galaxies, indicating rapid environmental impact.

Terminology

Environmental Quenching — 13× in this paper · explained in 2 episodes
AGN — 9× in this paper · explained in 1 episode
Velocity Dispersion — 9× in this paper · explained in 2 episodes
Benchmark — 5× in this paper · explained in 2 episodes
Metallicity — 4× in this paper · explained in 3 episodes
Aperture — 3× in this paper · explained in 1 episode
Population Synthesis — 3× in this paper · explained in 2 episodes
Core — 2× in this paper · explained in 2 episodes
Trace — 2× in this paper · explained in 2 episodes
Accuracy — 1× in this paper · explained in 1 episode
Active Galactic Nuclei (AGN) — 1× in this paper · explained in 1 episode
Adam — 1× in this paper · explained in 2 episodes

Summary

To probe how dense group environments affect galaxy evolution, this study observed molecular gas reservoirs in the over-dense group COSMOS-Gr30 at redshift z ∼ 0.7 using IRAM’s NOEMA and 30m telescopes to test models of environmental quenching.

How it works

The research focused on characterizing star formation regulation and quenching processes in a dense environment at intermediate redshift, specifically quantifying the molecular gas reservoirs within the group galaxies and investigating potential diffuse molecular gas within the intra-group medium. The target group, COSMOS-Gr30 (or CGr30), was selected for its high overdensity and its location at the intersection of large-scale cosmic web filaments. Deep MUSE observations revealed a large and massive structure of [O ii]λλ3726,3729-bright ionized gas extending over ∼ 104 kpc2 associated with eight group galaxies.

Key Observations and Findings

The study utilized CO(2-1) emission to probe molecular gas content in the galaxies. The results showed that group galaxies exhibit on average molecular gas contents reduced by ∼ 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. This suggests that environmental processes efficiently deplete molecular gas reservoirs in the galaxies of this group. Furthermore, the observations placed an upper limit on the molecular gas associated with the extended ionized structure at "Mgas < 2 × 1010 M⊙, implying that less than a third of the gas in the intra-group medium is in a cold, star-forming phase."

Molecular Gas Mass and Depletion Times

The analysis derived intrinsic CO(2-1) luminosities and molecular gas masses using conversion factors calibrated by PHIBSS2 methodology. The stacked measurements yielded µgas = 0.10+0.07−0.04 for both all galaxies and without AGN host, indicating that the average molecular gas mass is overall 20 − 40% of those found in typical star-forming galaxies. Correspondingly, the depletion time was found to be tdepl = 0.22+0.50−0.17 Gyr for all galaxies, which is compared to the median values found by Freundlich et al. (2019) between z = 0.5 − 0.8, namely tgdepl = 0.84±0.07 Gyr.

Comparison with Scaling Relations and Environmental Context

The molecular gas masses were compared against expected values derived from Tacconi et al. (2020) scaling relations, which express the molecular gas to stellar mass ratio as a function of redshift, stellar mass, and offset from the MS. The stacked molecular gas mass for all galaxies was found to be 0.20+0.43−0.15 of the expected value, indicating that the stacked measurements indicate an average trend for CGr30 galaxies to have lower molecular gas mass than the typical star-forming galaxies underlying the Tacconi et al. (2020) scaling relations. This depletion suggests that environmental processes in the group are highly efficient at removing or suppressing the molecular gas required for star formation, with possible mechanisms including tidal interactions, ram pressure stripping, and strangulation of the gas supply to the galaxies.

Role of Different Gas Tracers

The study utilized both CO(2-1) emission from NOEMA and 30m telescope observations to assess total molecular gas content. The complementary 30m observations yielded an upper limit for the total molecular gas emission in the beam, suggesting that less than a third of the gas displaced from the galaxies into the diffuse structure is in a cold phase. This simultaneous probing of galactic reservoirs and surrounding medium provides a consistent picture of environmental quenching, highlighting that processes like those causing depletion within galaxies are linked to the presence of extended, non-pristine gas structures. The findings underscore that environmental mechanisms in dense group environments act to remove or suppress molecular gas within galaxies, capturing quenching processes in action.

Conclusion and Future Directions

The results demonstrate that environmental processes in the CGr30 group are highly efficient at removing or suppressing the molecular gas required for star formation, typically down to 20 − 40% of the values observed in normal star-forming galaxies. The study concludes that the depletion of molecular gas within the galaxies, combined with the presence of an extended gas structure, provides a consistent picture of environmental quenching, but notes that these processes require further quantification through deeper observations with NOEMA, ALMA, and more precise SFR measurements. The authors also explored a preliminary trend between molecular gas fraction and distance to cosmic web filaments but stressed that significantly larger samples in well-studied fields such as COSMOS would be necessary to yield statistically significant results.

Improvements for AI systems

Based on the provided scientific paper, here are specific improvements for AI systems, focusing on areas where this research provides novel insights or robust data-driven methods:

The core value of this paper lies in its methodology for quantifying environmental quenching (molecular gas depletion) using multi-wavelength observations and scaling relations. The following AI improvements can be derived:

Enhanced Environmental Quenching Model Training (Supervised Learning/Regression):
Automated Gas Fraction Estimation via Multi-Tracer Fusion (Deep Learning/Fusion Networks):
Predictive Modeling of Galaxy Evolution Trajectories (Time-Series Forecasting):
Anomaly Detection in High-Redshift Environments (Unsupervised Learning/Clustering).

Specific Improvements and Capabilities:

AI System Improvement: Environmental Quenching Model Training (Supervised Learning/Regression)
Improved AI Capability: The system can be trained to predict the molecular gas fraction (µgas = Mgas/M⋆) or depletion time (tdepl = Mgas/SFR) of a galaxy based on its environment (group density, distance to cosmic web filaments, velocity dispersion).
AI System Improvement: Automated Gas Fraction Estimation via Multi-Tracer Fusion (Deep Learning/Fusion Networks)
Improved AI Capability: The system can ingest multi-wavelength observational data (e.g., CO(2-1) flux from NOEMA, [O II] luminosity from MUSE, stellar mass/SFR from SED models) for a galaxy and automatically derive the best estimate for its intrinsic molecular gas reservoir, explicitly accounting for the systematic offsets between different tracers (e.g., using the derived scaling relations like Eq. 4 or Fig. 6).
AI System Improvement: Predictive Modeling of Galaxy Evolution Trajectories (Time-Series Forecasting)
Improved AI Capability: The system can use the derived Star Formation Histories (SFH) for individual galaxies and group dynamical times to forecast future evolutionary states, predicting when a galaxy is likely to transition from starburst activity to quenching based on its current environmental interaction history.
AI System Improvement: Anomaly Detection in High-Redshift Environments (Unsupervised Learning/Clustering)
Improved AI Capability: The system can identify galaxies that deviate significantly from the established scaling relations (e.g., those falling far below the median expected molecular gas mass, as seen in Fig. 6), flagging them as potential outliers or systems undergoing rapid environmental processing, which could indicate unmodeled quenching mechanisms or unique physical states (like those potentially triggered by tidal interactions).

Abstract

We investigated the impact of group environment on molecular gas reservoirs at an intermediate redshift by observing the CO(2-1) emission from the galaxy group COSMOS-Gr30 at z about 0.7 with the Institut de Radioastronomie Millimetrique (IRAM) NOrthern Extended Millimeter Array (NOEMA) and 30m telescope. This dense environment, located at the intersection of large-scale cosmic web filaments hosts a large (about 10 4 kpc squared) ionized gas structure revealed by MUSE. We detect CO emission in four galaxies in the group with a signal-to-noise ratio (S/N) greater than five 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 about 0.5 dex relative to field scaling relations, corresponding to gas fractions that are 20- 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 gas < 2 times 10 10 M, implying that less than a third of the gas in the intragroup medium is in a cold, star-forming phase. Together, these results show how environmental mechanisms in dense group environments act to remove or suppress molecular gas within galaxies, capturing quenching processes in action.