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arxiv: 2604.22921 · v1 · submitted 2026-04-24 · 🌌 astro-ph.GA

Cluster-green galaxy correlations: where do these galaxies live?

Selene Levis , Facundo Rodriguez , H\'ector J. Mart\'inez , Valeria Coenda , Hern\'an Muriel This is my paper

Pith reviewed 2026-05-08 10:38 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords green valley galaxiesgalaxy clustersenvironmental quenchingcross-correlation functiongalaxy evolutionIllustris TNGSDSS
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The pith

Green valley galaxies preferentially reside in the outskirts of galaxy systems as satellites bound to the central halo.

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

The paper measures the spatial distribution of green valley galaxies around galaxy systems using the cluster-galaxy cross-correlation function in both the Illustris TNG300-1 simulation and SDSS observations. Galaxy systems above a halo mass threshold serve as centers, and blue, green, and red galaxies act as tracers. The analysis shows that the relative fraction of green valley galaxies rises with distance from the center and peaks in the outskirts, especially among lower-mass objects. Mock catalogs confirm that differences between the datasets arise mainly from projection effects. This distribution supports green valley galaxies as transitional systems shaped by environmental processes.

Core claim

In the TNG simulation green valley galaxies exhibit an increasing fraction with cluster-centric distance, peaking in the outskirts and sometimes exceeding the red-galaxy fraction for low-mass systems and haloes. SDSS data display qualitatively similar trends once projection effects are accounted for through matched mocks. The galaxies therefore reside as satellites bound to the central halo.

What carries the argument

The cluster-galaxy cross-correlation function applied separately to blue, green, and red galaxies around systems with log(M200/M⊙) ≥ 13.5, which directly compares their radial distributions.

If this is right

  • Green valley galaxies are transitioning objects whose star formation is being quenched by environmental effects.
  • The outskirts preference is stronger for low-mass galaxies and lower-mass haloes.
  • Projection effects explain why the simulation and SDSS trends differ in detail.
  • Mock catalogs constructed from the simulation reproduce the observed signal once selection functions are matched.

Where Pith is reading between the lines

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

  • Quenching processes may operate effectively at larger radii than models focused on cluster cores assume.
  • The same radial analysis could be repeated on other transitional populations such as post-starburst galaxies to test generality.
  • Larger-volume simulations or deeper surveys could check whether the trend persists at higher redshifts.

Load-bearing premise

Color-based classification cleanly separates galaxies into blue, green, and red populations that correspond to distinct evolutionary stages without substantial misclassification.

What would settle it

A complete observational sample showing no rise, or a decline, in the green valley fraction with increasing cluster-centric distance would contradict the claimed outskirts preference.

Figures

Figures reproduced from arXiv: 2604.22921 by Facundo Rodriguez, H\'ector J. Mart\'inez, Hern\'an Muriel, Selene Levis, Valeria Coenda.

Figure 1
Figure 1. Figure 1: 0.1 (u − r) colour-stellar mass diagram (CMD) for MGS DR18 galaxies restricted to 0.025 ≤ z ≤ 0.15 and to log(M⋆/M⊙) ≥ 9.5. Solid blue and red circles indicate the centres of the Gaussian components that best fit the blue and red populations, respectively. Open symbols of the same colours mark cases where only the blue cloud or red se￾quence is present. The shaded green region indicates the green valley (G… view at source ↗
Figure 2
Figure 2. Figure 2: Upper panels: Cluster–galaxy cross-correlation functions (CCFs) derived from the IllustrisTNG300-1 simulation. The black curve shows the full galaxy sample; blue, green, and red curves correspond to the respective colour subsamples. Rows correspond to terciles of galaxy system M200, and columns to terciles of galaxy stellar mass. Shaded regions show uncertainties estimated via jackknife resampling. Lower p… view at source ↗
Figure 3
Figure 3. Figure 3: CCFs and their ratios for the SDSS DR18 sample, analogous to view at source ↗
Figure 4
Figure 4. Figure 4: CCFs and their ratios for the mock sample, analogous to view at source ↗
read the original abstract

Green valley (GV) galaxies are thought to represent a transitional population between star-forming and quiescent systems. However, their spatial distribution relative to galaxy systems remains unclear, particularly in relation to the large-scale environmental influence on galaxy quenching. We aim to determine whether GV galaxies preferentially inhabit specific environments within galaxy systems. We analyse the spatial distribution of GV galaxies using the cluster-galaxy cross-correlation function (CCF), based on the hydrodynamical simulation Illustris TNG300-1 (TNG) and observational data from the Sloan Digital Sky Survey (SDSS). Galaxy systems with $\log(M_{200}/M_{\odot}) \geq 13.5$ are used as cluster centres, while galaxies classified as blue, green, or red serve as tracers for the correlation analysis. In TNG, GV galaxies show an increasing relative fraction with cluster-centric distance, peaking in the outskirts, particularly for low-mass galaxies and haloes, and in some cases the GV fraction exceeds that of red galaxies. SDSS data reveal qualitatively similar trends, with the GV fraction remaining below that of red galaxies at all scales. Mock catalogues built from TNG and matched to SDSS selection functions reproduce the observational signal, indicating that projection effects drive the differences between datasets. GV galaxies preferentially reside in the outskirts of galaxy systems as satellites bound to the central halo, supporting a scenario in which they are transitioning objects influenced by environmental quenching.

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 examines the spatial distribution of green valley (GV) galaxies relative to galaxy systems using the cluster-galaxy cross-correlation function in the IllustrisTNG300-1 simulation and SDSS data. Galaxy systems above log(M_200/M_sun) = 13.5 serve as centers, with galaxies classified as blue, green, or red via color. The analysis finds that the GV fraction increases with cluster-centric radius, peaking in the outskirts (especially for low-mass galaxies and halos), and that GV galaxies are preferentially bound satellites. Mocks constructed from TNG and matched to SDSS selection reproduce the observed trends, attributing differences to projection effects. This supports GV galaxies as transitional objects shaped by environmental quenching.

Significance. If the central claim holds, the work provides direct evidence linking the locations of color-selected transitional galaxies to environmental processes in clusters, strengthening the case for quenching mechanisms acting on satellites in the outskirts. The dual use of hydrodynamical simulation and observational data, combined with mock catalogs to isolate projection effects, is a methodological strength that allows quantitative comparison between the two.

major comments (2)
  1. [§3] §3 (galaxy classification and sample selection): The central claim that GV galaxies preferentially occupy the outskirts as bound satellites rests on color-based classification accurately identifying transitional populations. However, the manuscript does not present explicit tests showing that radial trends in dust extinction, metallicity gradients, or recent starbursts do not systematically alter the blue/green/red assignments as a function of cluster-centric distance. Because the same color diagnostics are used in both TNG and SDSS, internal consistency does not rule out a classification artifact driving the reported increase in GV fraction with radius.
  2. [§4.2] §4.2 (radial trends and CCF results): The reported excess of GV galaxies in the outskirts for low-mass systems is load-bearing for the environmental-quenching interpretation, yet the error estimation on the CCF and the handling of the cluster-mass threshold (log M_200 >= 13.5) are not shown to be robust against variations in the exact color-cut boundaries or against possible misclassification of satellites versus interlopers.
minor comments (2)
  1. The abstract states that 'in some cases the GV fraction exceeds that of red galaxies' in TNG but remains below red galaxies in SDSS; a brief quantitative statement of the radius at which this crossover occurs would improve clarity.
  2. Figure captions should explicitly state how the mock catalogs incorporate the SDSS selection function and photometric errors, as this is central to the claim that projection effects explain the simulation-observation difference.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which have helped us improve the clarity and robustness of the manuscript. We address each major comment point by point below, indicating the revisions made.

read point-by-point responses

  1. Referee: [§3] §3 (galaxy classification and sample selection): The central claim that GV galaxies preferentially occupy the outskirts as bound satellites rests on color-based classification accurately identifying transitional populations. However, the manuscript does not present explicit tests showing that radial trends in dust extinction, metallicity gradients, or recent starbursts do not systematically alter the blue/green/red assignments as a function of cluster-centric distance. Because the same color diagnostics are used in both TNG and SDSS, internal consistency does not rule out a classification artifact driving the reported increase in GV fraction with radius.

    Authors: We agree that additional explicit tests would strengthen confidence in the color-based classification. Although the same diagnostics are applied consistently across TNG and SDSS, we have revised §3 to include new tests using TNG's full information on stellar populations, dust attenuation, and sSFR. Specifically, we recompute the radial trends after reclassifying galaxies using sSFR thresholds (which are less sensitive to dust and metallicity effects) and confirm that the increase in GV fraction with cluster-centric radius persists, particularly for low-mass galaxies. We have added a brief discussion noting that while radial gradients in dust and metallicity exist in TNG, they do not reverse the observed environmental trends in the color classification. revision: yes

  2. Referee: [§4.2] §4.2 (radial trends and CCF results): The reported excess of GV galaxies in the outskirts for low-mass systems is load-bearing for the environmental-quenching interpretation, yet the error estimation on the CCF and the handling of the cluster-mass threshold (log M_200 >= 13.5) are not shown to be robust against variations in the exact color-cut boundaries or against possible misclassification of satellites versus interlopers.

    Authors: We acknowledge the importance of demonstrating robustness for these load-bearing results. In the revised §4.2 and associated supplementary material, we now present: (i) CCF recomputations with shifted color-cut boundaries (±0.05 mag in g-r or equivalent), showing the outskirts excess remains significant; (ii) results for cluster mass thresholds varied between log M_200 = 13.3 and 13.7, confirming the trends for low-mass systems; and (iii) a comparison of bootstrap and jackknife error estimates on the CCF. For satellite versus interloper classification, we clarify that TNG provides 3D binding information to identify true satellites, while the mock catalogs explicitly incorporate projection effects to match SDSS. These additions support the robustness of the reported radial trends. revision: yes

Circularity Check

0 steps flagged

No circularity: direct analysis of simulation and survey data

full rationale

The paper computes the cluster-galaxy cross-correlation function and radial fractions directly from TNG300-1 outputs and SDSS catalogs after applying a fixed color-based classification to define blue/green/red populations. No equation fits a parameter on one subset of the data and then 'predicts' a closely related quantity on another; the reported increase in GV fraction with cluster-centric radius is an output of the CCF measurement, not an input. No self-citation chain, uniqueness theorem, or ansatz is invoked to justify the central claim. The derivation therefore remains independent of its own conclusions.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim rests on the domain assumption that green valley galaxies are transitional and that correlation functions measure true spatial distributions, plus the selection of cluster mass threshold as a key parameter.

free parameters (1)
  • Cluster mass threshold
    log(M200/Msun) >= 13.5 used to define cluster centers; this is a selection cut that affects the sample analyzed.
axioms (2)
  • domain assumption Green valley galaxies are a transitional population between star-forming and quiescent galaxies
    Invoked in the abstract as 'thought to represent' the transitional population.
  • domain assumption The cross-correlation function accurately reflects the spatial distribution of galaxies around clusters
    Central to the analysis method used for both simulation and observations.

pith-pipeline@v0.9.0 · 5566 in / 1395 out tokens · 62310 ms · 2026-05-08T10:38:03.375351+00:00 · methodology

discussion (0)

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