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

¹ Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2A, Helsinki, FI-00014, Finland
² Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53, Avenue des Martyrs, 38000, Grenoble, France
³ Departamento de Astronomia, Instituto de Astronomia, Geofísica e Ciências Atmosféricas da USP, Cidade Universitária, 05508-900, São Paulo, SP, Brazil

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 October 2025
Accepted: 13 April 2026

Abstract

Context. Numerical simulations of hierarchical structure formation predict that galaxy clusters retain significant dark matter substructure, a signature of their ongoing assembly. This substructure is traced by both the spatial distribution of member galaxies and perturbations in the hot intracluster medium. Merging events significantly impact the thermodynamic state of clusters, introducing scatter in observable mass scaling relations and thereby affecting their use as precision cosmological probes.

Aims. 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.

Methods. 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. This provides a refined membership catalog and a classification of each cluster as containing substructure or not. 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.

Results. 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 L_X − λ relation. At low redshifts (z < 0.2), they display a systematically higher X-ray luminosity at fixed richness compared to relaxed systems.

Conclusions. We demonstrate that substructure identification with redMaPPer is viable and essential for enhancing the precision of cluster cosmology. 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.