The thermalisation of massive galaxy clusters

TL;DR

This paper investigates when massive galaxy clusters reach thermal equilibrium, finding it occurred around 1.8 Gyr ago at redshift 0.14, and shows that halo mass accretion history significantly influences this process.

Contribution

It provides the first observational estimate of the epoch of thermalisation in galaxy clusters and highlights the dominant role of mass accretion over radiative physics.

Findings

Massive clusters thermalised approximately 1.8 Gyr ago.

Kinetic energy from galaxy velocities traces gravitational energy over at least 5.4 Gyr.

Hot gas thermalisation efficiency increases with cosmic time.

Abstract

In the hierarchical scenario of structure formation, galaxy clusters are the ultimate virialised products in mass and time. Hot baryons in the intracluster medium (ICM) and cold baryons in galaxies inhabit a dark matter dominated halo. Internal processes, accretion, and mergers can perturb the equilibrium, which is established only at later times. However, the cosmic time when thermalisation is effective is still to be assessed. Here we show that massive clusters in the observed universe attained an advanced thermal equilibrium $\sim~1.8~\text{Gyr}$ ago, at redshift $z =0.14\pm0.06$, when the universe was $11.7\pm0.7~\text{Gyr}$ old. Hot gas is mostly thermalised after the time when cosmic densities of matter and dark energy match. We find in a statistically nearly complete and homogeneous sample of 120 clusters from the {\it Planck} Early Sunyaev-Zel'dovich (ESZ) sample that the kinetic energy traced by the galaxy velocity dispersion is a faithful probe of the gravitational energy since a look back time of at least $\sim5.4~\text{Gyr}$, whereas the efficiency of hot gas in converting kinetic to thermal energy, as measured through X-ray observations in the core-excised area within $r_{500}$, steadily increases with time. The evolution is detected at the $\sim 98$ per cent probability level. Our results demonstrate that halo mass accretion history plays a larger role for cluster thermal equilibrium than radiative physics. The evolution of hot gas is strictly connected to the cosmic structure formation.