Timing the last major merger of galaxy clusters with large halo sparsity

TL;DR

This paper introduces a new method using halo sparsity measurements to detect and time major galaxy cluster mergers, validated with simulations and applied to real clusters, enabling insights into cluster assembly history.

Contribution

The paper presents a novel statistical approach and a public Python library to identify and date major mergers in galaxy clusters based on halo sparsity evolution.

Findings

Major mergers leave a universal imprint on halo sparsity.

The method can estimate merger timing within two dynamical times.

Applied to real clusters, it identified a merger in Abell 2345 about 2.1 Gyr ago.

Abstract

Numerical simulations have shown that massive dark matter haloes, which today host galaxy clusters, assemble their mass over time alternating periods of quiescent accretion and phases of rapid growth associated with major merger episodes. Observations of such events in clusters can provide insights on the astrophysical processes that characterise the properties of the intra-cluster medium, as well as the gravitational processes that contribute to their assembly. It is therefore of prime interest to devise a fast and reliable way of detecting such perturbed systems. We present a novel approach to identifying and timing major mergers in clusters characterised by large values of halo sparsity. Using halo catalogues from the MultiDark-Planck2 simulation, we show that major merger events disrupt the radial mass distribution of haloes, thus leaving a distinct universal imprint on the evolution of halo sparsity over a period not exceeding two dynamical times. We exploit this feature using numerically calibrated distributions to test whether an observed galaxy cluster with given sparsity measurements has undergone a recent major merger and to eventually estimate when such an event occurred. We implement these statistical tools in a specifically developed public python library \textsc{lammas}, which we apply to the analysis of Abell 383 and Abell 2345 as test cases. Finding that, for example, Abell 2345 had a major merger about $2.1\pm 0.2$ Gyr ago. This work opens the way to detecting and timing major mergers in galaxy clusters solely through measurements of their mass at different radii.