The Dynamical State and Mass-Concentration Relation of Galaxy Clusters

TL;DR

This study uses simulations to show that many massive galaxy clusters are observed during transient, non-equilibrium phases, affecting their measured properties and challenging simple equilibrium-based models.

Contribution

It reveals that the mass-concentration relation's upturn in massive halos is due to their dynamical state, emphasizing the need to consider non-equilibrium conditions in cluster analyses.

Findings

Massive halos often observed out of equilibrium during transient phases.

The upturn in the mass-concentration relation is linked to non-equilibrium states.

Relaxed systems do not show the upturn, highlighting the importance of dynamical state.

Abstract

We use the Millennium Simulation series to study how the dynamical state of dark matter halos affects the relation between mass and concentration. We find that a large fraction of massive systems are identified when they are substantially out of equilibrium and in a particular phase of their dynamical evolution: the more massive the halo, the more likely it is found at a transient stage of high concentration. This state reflects the recent assembly of massive halos and corresponds to the first pericentric passage of recently-accreted material when, before virialization, the kinetic and potential energies reach maximum and minimum values, respectively. This result explains the puzzling upturn in the mass-concentration relation reported in recent work for massive halos; indeed, the upturn disappears when only dynamically-relaxed systems are considered in the analysis. Our results warn against applying simple equilibrium models to describe the structure of rare, massive galaxy clusters and urges caution when extrapolating scaling laws calibrated on lower-mass systems, where such deviations from equilibrium are less common. The evolving dynamical state of galaxy clusters ought to be carefully taken into account if cluster studies are to provide precise cosmological constraints.