The signature of major mergers on the hydrostatic mass bias of galaxy clusters

¹ Departament d’Astronomia i Astrofísica, Universitat de València 46100 Burjassot, Spain
² Dipartimento di Fisica e Astronomia, Università di Bologna Via Gobetti 93/2 IT-40129 Bologna, Italy
³ Observatori Astronòmic, Universitat de València 46980 Paterna (València), Spain

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

Received: 25 July 2025
Accepted: 13 November 2025

Abstract

Context. While the masses and abundances of galaxy clusters are key observables for Cosmology, mass determinations based on intra-cluster medium observations often rely on the hydrostatic equilibrium assumption. This assumption introduces a systematic bias that is expected to be more noticeable during strong assembly episodes, such as major mergers.

Aims. We aim to study in detail how mergers shape the variability in the hydrostatic mass bias values through the evolutionary history of clusters, identify the primary mechanisms driving such evolution, and assess its potential dependence on dynamical state and merging history.

Methods. Using a moderate-sized adaptive mesh refinement Eulerian plus N-Body cosmological simulation, we identified a sample of galaxy cluster mergers in the redshift interval 1.5 ≥ z ≥ 0. We compared true and hydrostatic masses within the virial volume. We derived the latter from gas density and temperature radial profiles. The evolution was assessed in relation to the merging history extracted from halo merger trees.

Results. At z = 0, the hydrostatic mass bias shows a mild correlation with dynamical state. During major mergers, the bias follows a characteristic trend: a sharp negative dip around the merger time, a transient positive peak, and a gradual return to pre-merger levels. This behaviour is primarily driven by morphological and dynamical reconfigurations of the gas density within the ICM, while thermodynamic processes play a secondary role. The pattern shows no strong dependence on secondary parameters, such as mass ratio or impact parameter, but it can be fitted to a simple time-dependent functional form. This trend is present at radii r ≤ R_vir, although with reduced amplitude and shorter timescales as the radius decreases.

Conclusions. Hydrostatic mass bias is closely linked, albeit in a non-trivial way, with the merging history of galaxy clusters. We find that the bias values are weakly correlated with the dynamical state of clusters. Nevertheless, our results give a robust estimation of the hydrostatic mass bias values in the pre-merger, merging, and post-merger phases. These findings highlight the importance of delving deeper into the observational assessment of cluster assembly state in order to improve mass estimations for cosmological analyses.