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

TL;DR

This study uses high-resolution cosmological simulations to analyze how major galaxy cluster mergers influence the hydrostatic mass bias, revealing characteristic temporal patterns and their dependence on dynamical state and merger history.

Contribution

It provides a detailed analysis of the physical mechanisms driving hydrostatic mass bias during mergers, including a simple functional form to model its evolution over time.

Findings

Hydrostatic mass bias exhibits a characteristic trend during mergers: a negative dip, a positive peak, then a return to pre-merger levels.

Morphological and dynamical reconfigurations of the gas density primarily drive the bias evolution.

The bias pattern is consistent across different radii within the virial radius, with variations in amplitude and timescale.

Abstract

While galaxy cluster masses are fundamental cosmological observables, estimates based on intra-cluster medium observations rely on hydrostatic equilibrium, introducing a systematic bias. We investigate how mergers drive the time evolution of this hydrostatic mass bias, identifying the dominant physical mechanisms and their dependence on dynamical state and merger history. Using a high-resolution AMR Eulerian+$N$-body cosmological simulation, we analyse a sample of cluster mergers within $1.5 \leq z \leq 0$, comparing true and hydrostatic masses derived from gas density and temperature profiles, and tracing their evolution. 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 thermodynamical 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 $r\le R_{\mathrm{vir}}$, although with reduced amplitude and shorter timescales as the radius decreases. 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.