Substructure in redMaPPer clusters and its impact on X-ray morphology and scaling relations

A. Finoguenov; J. Comparat; L. Doubrawa; R. Tuomainen

Substructure in redMaPPer clusters and its impact on X-ray morphology and scaling relations

T0 review · 3 major / 2 minor · reviewed 2026-05-07 · grok-4.3

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Pith's one-line read Substructure is present in 40% of redMaPPer clusters, correlates with disturbed X-ray morphology, and drives redshift-dependent scatter in the Lx-lambda relation, with stronger effects at low z.

desk verdict The paper finds ~40% of redMaPPer clusters have HDBSCAN-detected substructure that correlates with disturbed eROSITA X-ray morphologies and drives a low-z Lx offset at fixed richness, but the substructure label may include projections. read the letter →

arxiv 2604.25770 v1 submitted 2026-04-28 astro-ph.CO astro-ph.HE

R. Tuomainen , A. Finoguenov , J. Comparat , L. Doubrawa This is my paper · ORCID

classification astro-ph.COastro-ph.HE

keywords x-raysubstructureclustersmorphologyopticalrichnessscalingcatalog

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Galaxy clusters contain hot gas that glows in X-rays and galaxies that can be counted from the ground. The redMaPPer catalog selects clusters by their red galaxies. Researchers ran the HDBSCAN clustering algorithm on the positions and membership probabilities of those galaxies to spot smaller groups inside the main cluster. They found substructure in roughly 40% of clusters, with a quarter showing more than 35% of the total galaxy count coming from the sub-group. They then matched these clusters to X-ray data from the eROSITA satellite. Clusters with optical substructure showed more disturbed X-ray shapes, especially the most massive ones. These same clusters also produced more scatter in the relation between X-ray brightness and galaxy count, and at low redshifts they appeared brighter in X-rays for a given galaxy count. The authors suggest that cool cores are more prominent nearby and that substructures survive longer at low redshift. At lower galaxy counts, X-ray shape sometimes disagrees with the optical merging signal, hinting that active galactic nuclei feedback becomes more important.

Extended reading notes

Core claim

We find a highly significant correlation between optical substructure and disturbed X-ray morphologies, a trend that is strongest for high-mass clusters. The clusters with substructure also drive a stronger redshift evolution in the scatter of the Lx-lambda relation. At low redshifts (z<0.2), they display a systematically higher X-ray luminosity at fixed richness compared to relaxed systems.

Load-bearing premise

That HDBSCAN applied to probabilistic redMaPPer memberships reliably identifies physically merging substructures and that the optical-X-ray cross-match is complete and unbiased across richness and redshift.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The core result is that substructure appears in about 40% of the sample, with a quarter showing more than 35% of the richness in secondary components, and this splits the X-ray morphology and scaling relations in a mass- and redshift-dependent way. Clusters flagged as substructured show more disturbed X-ray shapes, stronger scatter evolution in the Lx-lambda relation, and higher luminosity at fixed richness below z=0.2. The authors link the low-redshift amplification to cool-core contrast and longer merger survival times, possibly aided by magnetic draping, while noting that AGN feedback may dominate morphology at lower richness. This is incremental but useful: it applies an established density-based algorithm to an existing optical catalog and cross-matches it with the new all-sky eROSITA data to produce concrete prevalence numbers and trend splits. The observational correlations are cleanly presented and the low-z luminosity offset is a clear, testable claim. The main limitation is that HDBSCAN is run directly on redMaPPer probabilistic memberships without reported mock-catalog tests for line-of-sight contamination or purity. Photometric redshifts make projection effects plausible, and any richness- or redshift-dependent incompleteness in the X-ray cross-match could bias the reported trends. The abstract calls the correlations highly significant but supplies no sample sizes, error bars, or selection-function details, so the strength is difficult to judge without the full methods. The physical attributions are plausible but rest on the substructure label being reliable. This work is for people building or calibrating cluster scaling relations for cosmology, especially those using X-ray or optical richness proxies at low redshift. Readers who need to know how merger state affects observables will extract practical numbers from it. The paper is coherent on its own terms and brings fresh cross-matched data, so it deserves a serious referee even if the substructure validation needs tightening. I would send it to review.

Referee Report

3 major / 2 minor

Summary. The paper applies the HDBSCAN density-based clustering algorithm to probabilistic galaxy membership probabilities from the redMaPPer catalog (using DECaLS data) to identify substructure, reporting its presence in ~40% of clusters with a quarter showing >35% fractional richness contribution. It cross-matches the sample with the eROSITA X-ray morphology catalog and finds a highly significant correlation between optical substructure and disturbed X-ray morphologies (strongest in high-mass clusters). Substructured clusters are shown to drive stronger redshift evolution in the scatter of the L_X-λ scaling relation and, at z<0.2, systematically higher L_X at fixed richness; the authors attribute these effects to cool-core contrast and magnetic draping, while noting a growing role for AGN feedback at lower richness.

Significance. If the central correlations hold after validation, the work supplies a large-sample observational link between independently measured optical substructure and X-ray morphology/scaling relations, with direct implications for the use of cluster scaling relations in cosmology. The application of HDBSCAN to probabilistic memberships and the use of new eROSITA all-sky data are positive technical features that enable the mass- and redshift-dependent trends reported.

major comments (3)

[Substructure detection section] The substructure detection section: HDBSCAN applied to redMaPPer P(mem) values is not validated with mock catalogs, N-body simulations, or spectroscopic follow-up to demonstrate that the detected overdensities correspond to physically bound merging substructures rather than line-of-sight projections or noise. This assumption is load-bearing for the headline claims of a significant optical-X-ray morphology correlation and the L_X-λ trends, because projection biases could preferentially affect the reported mass dependence and low-z behavior.
[eROSITA cross-match section] The eROSITA cross-match section: completeness and selection biases of the optical-X-ray cross-match are not quantified as functions of richness, redshift, or morphology. Any richness- or z-dependent incompleteness that correlates with disturbed vs. relaxed states would directly affect the reported correlations and the split-sample scaling-relation results.
[Scaling relations section] The scaling-relation analysis: the claim that substructured clusters drive stronger redshift evolution in L_X-λ scatter and higher L_X at fixed λ (z<0.2) requires explicit reporting of sample sizes per bin, covariance with mass/redshift, and tests that the trends survive after accounting for possible projection or selection effects identified in the two preceding points.

minor comments (2)

[Abstract] Abstract: sample size, exact statistical significance (p-values or equivalent), and uncertainties on the 40% fraction and scaling-relation offsets should be stated explicitly.
[Methods] Notation: the definition and threshold used for 'substructure' (e.g., minimum cluster size in HDBSCAN, fractional richness cut) should be stated once in the main text with a clear reference to the methods.

Assumptions & free parameters 0 free parameters · 2 assumptions · 0 invented entities

Based on abstract only; no explicit free parameters, invented entities, or non-standard axioms are stated. The analysis rests on standard assumptions about cluster membership probabilities and X-ray morphology metrics.

assumptions (2)

domain assumption HDBSCAN on redMaPPer probabilistic memberships identifies physically meaningful substructure
Central to the optical substructure measurement described in the abstract
domain assumption Cross-matched eROSITA X-ray morphology parameters are unbiased tracers of dynamical state
Required for the claimed correlation between optical substructure and X-ray disturbance

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Cite this review

Pith. "Pith review of Substructure in redMaPPer clusters and its impact on X-ray morphology and scaling relations." pith.science (2026). https://pith.science/paper/2604.25770

@misc{pith2026260425770,
  author       = {Pith},
  title        = {Pith review of: Substructure in redMaPPer clusters and its impact on X-ray morphology and scaling relations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/2604.25770}},
  note         = {Machine review of arXiv:2604.25770}
}

read the original abstract

We statistically quantified the prevalence and properties of substructure in optical galaxy clusters and directly investigated its impact on X-ray morphology and scaling relations, leveraging new data from the DECaLS Legacy Survey and the SRG/eROSITA all-sky survey. We applied the hierarchical density-based clustering algorithm HDBSCAN to the redMaPPer galaxy cluster catalog to identify and characterize substructure from the probabilistic membership assignments. We then cross-matched this sample with the eROSITA X-ray morphology catalog to correlate optical substructure with a comprehensive set of X-ray morphological parameters. Finally, we analyzed the scaling relation between X-ray luminosity and optical richness for clusters with and without substructure. Substructure is a common feature, present in approximately 40% of clusters; a quarter of the full sample exhibits a fractional contribution to richness in excess of 35%. We find a highly significant correlation between optical substructure and disturbed X-ray morphologies, a trend that is strongest for high-mass clusters. The clusters with substructure also drive a stronger redshift evolution in the scatter of the Lx-lambda relation. At low redshifts (z<0.2), they display a systematically higher X-ray luminosity at fixed richness compared to relaxed systems. We attribute the enhanced effect of mergers on X-ray properties at low redshifts to the increased density contrast of low-redshift cool cores and longer substructure survival times, which are possibly due to the suppression of disruptive mixing by effects such as magnetic draping. At lower cluster richness, a discordance between X-ray morphology and the merging state indicates a growing relative importance of active galactic nucleus feedback in governing X-ray morphology.

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Reviewed May 7, 2026 · model on record in the stance chip above.