The dynamical state of eROSITA clusters and its impact on the brightest cluster galaxy luminosity

¹ Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile
² Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
³ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁴ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁵ Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
⁶ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
⁷ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
⁸ Astronomy Unit, Department of Physics, University of Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁹ INFN – National Institute for Nuclear Physics, Via Valerio 2, I-34127 Trieste, Italy
¹⁰ ICSC – Italian Research Center on High Performance Computing, Big Data and Quantum Computing, Bologna, Italy
¹¹ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan
¹² International Gemini Observatory/NSF NOIRLab, Casilla 603, La Serena, Chile
¹³ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111, Valparaíso, Chile
¹⁴ Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Valparaíso, Chile
¹⁵ Departamento de Astronomía, Universidad de La Serena, Av. Raúl Bitrán 1305, La Serena, Chile
¹⁶ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
¹⁷ Department of Physics and Astronomy, University of Wyoming, 1000 E. University, Dept. 3905, Laramie, WY 82071, USA
¹⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁹ NSF’s National Optical/Infrared Research Laboratory (NOIRLab), 950 N. Cherry Ave, Tucson AZ 85732, USA
²⁰ Universität Innsbruck, Institut für Astro-und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria

^⋆ Corresponding author: alfredo.zenteno@noirlab.edu

Received: 1 October 2024
Accepted: 26 March 2025

Abstract

Context. The first Spectrum-Roentgen-Gamma (SRG) eROSITA public release contains 12 247 clusters and groups from its first 6 months of operation. We used the offset between the brightest cluster galaxy (BCG) and the X-ray peak (D_{BCG − X}) to classify the cluster dynamical state of 3946 galaxy clusters and groups.

Aims. We aim to investigate the evolution of the merger and relaxed cluster distributions with redshift and mass, and the distributions’ impact on the BCG.

Methods. We used the X-ray peak from the eROSITA survey and the BCG position from the LS DR10 optical data, which includes the DECam eROSITA Survey optical data, to measure the D_{BCG − X} offset. We modelled the distribution of D_{BCG − X}, in units of R₅₀₀, as the sum of two Rayleigh distributions representing the cluster’s relaxed and disturbed populations, and explored their evolution with redshift and mass. To explore the impact of the cluster’s dynamical state on the BCG luminosity, we separated the main sample according to the dynamical state. We defined clusters as relaxed if D_{BCG − X} < 0.25R₅₀₀, disturbed if D_{BCG − X} > 0.5R₅₀₀, and ‘diverse’ if 0.25R₅₀₀ < D_{BCG − X} < 0.5R₅₀₀.

Results. We find no evolution of the merging fraction with redshift or mass. The width of the relaxed distribution increases with redshift, while the width of the two Rayleigh distributions decreases with mass. The analysis reveals that BCGs in relaxed clusters are brighter than BCGs in both the disturbed and diverse cluster populations. The most significant differences are found for high-mass clusters at higher redshifts.

Conclusions. The results suggest that BCGs in low-mass clusters are less centrally bound than those in high-mass systems, irrespective of the dynamical state. Over time, BCGs in relaxed clusters progressively align with the potential centre. This alignment correlates with their luminosity growth relative to BCGs in dynamically disturbed clusters, underscoring the critical role of the cluster’s dynamical state in regulating BCG evolution.