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arxiv: 2604.08402 · v1 · submitted 2026-04-09 · 🌌 astro-ph.GA

Neutral Hydrogen in the Shapley Supercluster Core I: Environmental Effects on Gas Content and Galaxy Evolution

L. Gwebushe , T. Venturi , P. Merluzzi , G. Busarello , V. Casasola , O. Smirnov , M. Ramatsoku , J. Dawson This is my paper

Pith reviewed 2026-05-10 17:26 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords neutral hydrogenShapley Superclusterenvironmental quenchinggalaxy evolutionstar formation main sequenceHI depletion timescaleMeerKAT survey
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The pith

Galaxies in the Shapley Supercluster core quench via gas starvation rather than rapid stripping.

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

The paper measures atomic hydrogen in 169 galaxies within the active core of the Shapley Supercluster and compares their gas content to field samples. Star-forming galaxies sit near the expected gas fraction, but transition and red-sequence galaxies show clear depletions, with depletion timescales stretching from 6 to 170 billion years. A much higher share of transition galaxies appears than in the field, and star-forming and red galaxies have similar sizes, both unlike field trends. These patterns indicate that the dense environment cuts off fresh gas supply, slowing star formation gradually instead of stripping existing gas quickly.

Core claim

Galaxies on the star-forming main sequence in the SSC core show only a small 0.038 dex offset from the gas-fraction main sequence, while transition galaxies sit at -0.034 dex and red-sequence galaxies at -0.211 dex; HI depletion times increase from 6 Gyr to 170 Gyr across these classes. Scaling relations between HI mass and stellar mass remain close to field values, yet the population is dominated by transition galaxies rather than star-forming ones, and star-forming and red-sequence galaxies have comparable sizes. These observations indicate environmental quenching proceeds through starvation or strangulation, with detectable HI reservoirs but reduced accretion and inefficient star-steady.

What carries the argument

Offsets from the HI gas-fraction main sequence and HI depletion timescales measured separately for star-forming main sequence, transition-zone, and red-sequence populations, compared against a field reference sample.

If this is right

  • Transition galaxies become the dominant population in the dense SSC core, unlike the star-forming majority seen in field samples.
  • Star-forming and red-sequence galaxies in the SSC have similar sizes, reversing the size trend observed in the field.
  • HI depletion timescales lengthen steadily from star-forming through transition to red-sequence galaxies, confirming progressively less efficient star formation.
  • HI-to-stellar-mass scaling relations stay broadly consistent with field samples even inside the supercluster.

Where Pith is reading between the lines

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

  • The same starvation process may operate in other nearby supercluster cores where dynamical activity is high but direct stripping is not yet dominant.
  • Long depletion times despite retained HI suggest that reduced external accretion, rather than internal consumption, drives the observed slowdown in star formation.
  • Targeted HI mapping of additional transition galaxies in other clusters could test whether the predominance of this class is a general signature of gradual environmental quenching.

Load-bearing premise

The division of galaxies into star-forming, transition, and red-sequence classes from optical data correctly tracks their gas consumption and evolutionary stage, and the field comparison sample matches the SSC sample in selection properties.

What would settle it

Finding frequent HI tails, asymmetries, or truncated disks in a majority of the detected galaxies would indicate rapid stripping instead of starvation.

Figures

Figures reproduced from arXiv: 2604.08402 by G. Busarello, J. Dawson, L. Gwebushe, M. Ramatsoku, O. Smirnov, P. Merluzzi, T. Venturi, V. Casasola.

Figure 1
Figure 1. Figure 1: Map of the numerical surface density of SSC galaxies as identified by spectroscopic redshift in the Shapley Supercluster Survey region (Merluzzi et al. 2015). The 11 clusters within the area are identified and marked with black circles, with radii corresponding to R200. The white circles indicate the two MeerKAT Hi pointings analysed in this paper. Stokes-I imaging was performed using MFImage, which correc… view at source ↗
Figure 2
Figure 2. Figure 2: Mosaic showing the total intensity (Hi) images of detected sources in the SC1329-313 and Abell 3558 fields, with flux density represented in standard units. The dashed-dotted circles indicate the primary FoV ∼ 50′ for each MeerKAT pointing. The centres of the two pointings are marked by pink stars [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Image showing the spatial distribution of Hi-detected sources after applying the dynamical analysis used to assign each source to its host structure. Blue circles indicate sources associated with Abell 3558, green circles with SC1329-313, and cyan circles with Abell 3562. Field 0 (yellow circles) and SSC 1 (black circles) are identified as galaxies that belong to the SSC structure but are not within R200 o… view at source ↗
Figure 4
Figure 4. Figure 4: Stellar mass distribution of galaxies with detected Hi. The orange bars represent the SSC-ShaSS sample of Hi-detected galaxies, while red unfilled histogram represent SSC-ShaSS complete sample of Hi-detected galaxies. to account for factors affecting stellar mass estimates, such as the assumed initial mass function (IMF). The stellar masses in our full sample span the range 𝑀★ ∼ 3 × 108−2×1011 M⊙ at redshi… view at source ↗
Figure 5
Figure 5. Figure 5: Hi mass distribution of detected galaxies. The left panel shows the distribution of Hi masses for the full sample of galaxies detected across both MeerKAT pointings. The right panel displays the Hi mass distribution for galaxies associated with the SSC-ShaSS sample, specifically those belonging to Abell 3558 and SC1329, and a red unfilled histogram show the SSC-ShaSS complete sample. The SSC-ShaSS complete… view at source ↗
Figure 6
Figure 6. Figure 6: Left: Specific star formation rate (sSFR300) versus stellar mass for Hi-detected galaxies in the Abell 3558 and SC1329 region, colour-coded by ΔSFMS population: star-forming main sequence (SFMS; blue), transition zone (TZ; green), and red sequence (RS; brown). The solid black line indicates the empirical star-forming main sequence (SFMS), with dotted lines representing a scatter of ±0.3 dex. Right: Hi gas … view at source ↗
Figure 7
Figure 7. Figure 7: Distributions of Hi properties for galaxies in the SSC-core region, classified by gas content and position relative to the star-forming main sequence (SFMS). In all three panels, horizontal bars at the top of each distribution indicate the mean, 25th percentile, and 75th percentile values for each population. Left: Distribution of ΔSFMS for galaxies in the gas-rich (GR; orange), gas-normal (GN; magenta), a… view at source ↗
Figure 8
Figure 8. Figure 8: The figure shows the average Hi gas fraction per bin as a function of galaxy stellar mass in the left panel and the average Hi mass per bin as a function of galaxy stellar mass in the right panel. The red diamonds represent our sample (SSC-core) with the red solid line denotes a linear fit to the sample. Grey pentagons and cyan down-triangles correspond to xGASS detections and 5𝜎 upper limits for non-detec… view at source ↗
read the original abstract

We study the atomic Hydrogen (HI) content of galaxies in the core of the Shapley Supercluster (SSC) at <z> ~ 0.048, using observations from the MeerKAT Galaxy Cluster Legacy Survey and optical data from the Shapley Supercluster Survey (ShaSS) project. Our sample comprises 169 galaxies with HI detections in the dynamically active region of Abell 3558 and SC1329-313. Following the literature, we classify galaxies into star-forming main sequence (SFMS), transition (TZ), and red sequence (RS) populations, and examine how the HI content varies across these populations. Galaxies on the SFMS exhibit an average HI gas fraction offset of 0.038 dex from the gas fraction main sequence, while TZ and RS populations show depleted HI fractions of -0.034 and -0.211 dex. HI depletion timescales span from 6 to 170 Gyr (SFMS-TZ-RS) confirming increasingly inefficient star formation with quenching. Scaling relations between HI mass and stellar mass in the SSC are generally consistent with field samples. The most direct signature of the dense environment of the SSC is the marked predominance of TZ galaxies, in contrast to what is observed in the field-dominated sample of xGASS, where the population is mostly composed of SFMS galaxies. Moreover, the SFMS and RS populations have similar size, again in contrast with field populations. These results suggest that galaxies in the SSC are undergoing environmental quenching through starvation or strangulation, rather than rapid gas stripping. Despite detectable HI reservoirs, many galaxies exhibit long depletion times, indicating reduced gas accretion and inefficient star formation.

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 / 3 minor

Summary. The manuscript presents MeerKAT HI observations of 169 galaxies in the dynamically active core of the Shapley Supercluster (Abell 3558 and SC1329-313) at <z> ~ 0.048, combined with optical classifications from the Shapley Supercluster Survey. Galaxies are divided into star-forming main sequence (SFMS), transition (TZ), and red sequence (RS) populations. Reported HI gas-fraction offsets relative to field expectations are +0.038 dex (SFMS), -0.034 dex (TZ), and -0.211 dex (RS), with depletion timescales ranging from 6 to 170 Gyr across these populations. M_HI–M_* scaling relations are described as generally consistent with field samples, yet the SSC sample shows a marked excess of TZ galaxies and similar sizes for SFMS and RS galaxies compared with the xGASS field sample. The authors conclude that environmental quenching proceeds via starvation or strangulation rather than rapid stripping.

Significance. If the reported population contrasts prove robust, the work supplies direct HI constraints on quenching mechanisms in a supercluster core using new MeerKAT data. The large number of HI detections, the multi-population analysis, and the explicit contrast with a field reference sample constitute clear strengths. The results would usefully inform models of slow environmental quenching and complement existing cluster HI studies.

major comments (2)
  1. The headline claim that the SSC environment drives quenching via starvation/strangulation (abstract and final paragraph) rests on the excess of optically classified TZ galaxies relative to xGASS. The manuscript does not state whether the stellar-mass distributions of the 169 HI-detected SSC members and the xGASS comparison sample were matched, nor does it report the parent-sample completeness or selection functions. Because the TZ fraction is known to vary with stellar mass, an unmatched comparison risks conflating selection with environment. A mass-matched re-analysis or explicit mass histograms for both samples is required to substantiate the inference.
  2. The reported dex offsets in HI gas fraction and the depletion-time range (abstract) are presented without uncertainties, without specification of the exact reference field relation used for the offsets, and without discussion of how MeerKAT data reduction and source extraction affect the measured values. These quantities underpin the statement that star formation becomes increasingly inefficient from SFMS to RS; their statistical significance and robustness must be quantified.
minor comments (3)
  1. The abstract states that M_HI–M_* scaling relations are 'generally consistent' with field samples but does not identify the precise reference relation or quantify the level of consistency (e.g., via residuals or a dedicated figure).
  2. All acronyms (SFMS, TZ, RS, xGASS, ShaSS) should be defined on first use in the main text, even if already introduced in the abstract.
  3. The optical classification criteria that assign galaxies to SFMS, TZ, and RS populations should be stated explicitly, including any adopted SFR or color thresholds and how they were applied to the ShaSS photometry.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their positive summary of our work and for the constructive major comments, which have identified important areas for clarification and strengthening. We address each point below and have revised the manuscript accordingly to incorporate additional analyses, figures, and discussions.

read point-by-point responses

  1. Referee: The headline claim that the SSC environment drives quenching via starvation/strangulation (abstract and final paragraph) rests on the excess of optically classified TZ galaxies relative to xGASS. The manuscript does not state whether the stellar-mass distributions of the 169 HI-detected SSC members and the xGASS comparison sample were matched, nor does it report the parent-sample completeness or selection functions. Because the TZ fraction is known to vary with stellar mass, an unmatched comparison risks conflating selection with environment. A mass-matched re-analysis or explicit mass histograms for both samples is required to substantiate the inference.

    Authors: We agree that the manuscript did not explicitly address stellar-mass matching or report completeness and selection functions, which is a valid concern given the known mass dependence of the TZ fraction. In the revised version we will add stellar-mass histograms comparing the SSC HI-detected sample with the xGASS reference sample and will include a mass-matched re-analysis of the population fractions. We will also expand the methods section to describe the parent-sample completeness, the HI detection limits, and the optical selection from the ShaSS survey. These additions will allow a direct assessment of whether the observed excess of TZ galaxies is attributable to environment rather than selection effects. revision: yes

  2. Referee: The reported dex offsets in HI gas fraction and the depletion-time range (abstract) are presented without uncertainties, without specification of the exact reference field relation used for the offsets, and without discussion of how MeerKAT data reduction and source extraction affect the measured values. These quantities underpin the statement that star formation becomes increasingly inefficient from SFMS to RS; their statistical significance and robustness must be quantified.

    Authors: We acknowledge that the abstract and main text did not provide uncertainties on the reported offsets, did not name the precise reference relation, and did not discuss potential systematics from the MeerKAT pipeline. The offsets are computed relative to the HI gas-fraction main sequence defined by the xGASS survey. In the revised manuscript we will quote the uncertainties on all dex offsets and on the depletion timescales (obtained via bootstrap resampling), explicitly cite the xGASS reference relation, and add a paragraph in the methods section describing the impact of the MeerKAT data reduction, SoFiA source extraction, and interferometer flux recovery on the measured HI masses. These changes will quantify the statistical significance and observational robustness of the trend toward longer depletion times from SFMS to RS. revision: yes

Circularity Check

0 steps flagged

No circularity: claims rest on external sample comparisons

full rationale

The paper's chain consists of HI detections from MeerKAT, optical classification of SFMS/TZ/RS populations following standard literature definitions, computation of gas-fraction offsets and depletion times directly from the observed data, and direct comparison of population fractions and scaling relations to the independent xGASS field sample. No parameters are fitted to a subset and then re-predicted, no self-definitional loops exist in the classification or scaling relations, and no load-bearing premises reduce to self-citations. The central inference (environmental quenching via starvation) is presented as an interpretation of the observed excess of TZ galaxies relative to the external benchmark, which remains falsifiable by future matched samples.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work relies on standard assumptions in HI astronomy and galaxy classification rather than new postulates.

axioms (2)
  • standard math HI mass is correctly derived from 21-cm line flux using standard conversion factors and distance assumptions at z~0.048.
    Invoked implicitly in all HI mass and gas fraction calculations.
  • domain assumption Optical classification into SFMS, TZ, and RS populations using ShaSS data accurately separates evolutionary stages.
    Central to interpreting HI content differences across populations.

pith-pipeline@v0.9.0 · 5641 in / 1288 out tokens · 38833 ms · 2026-05-10T17:26:48.348187+00:00 · methodology

discussion (0)

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Reference graph

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5 extracted references · 5 canonical work pages

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