The thermalization of massive galaxy clusters

In the hierarchical scenario of structure formation, galaxy clusters are the ultimate virialized 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 thermalization is effective is still to be assessed. Here, we show that massive clusters in the observed universe attained an advanced thermal equilibrium similar to 1.8 Gyr ago, at redshift z = 0.14 +/- 0.06, when the universe was 11.7 +/- 0.7 Gyr old. Hot gas is mostly thermalized 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 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 similar to 5.4 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 similar to 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.

Automated summary

Research shows that massive galaxy clusters in the observed universe achieved an advanced thermal equilibrium around 1.8 billion years ago, influenced by the matching densities of matter and dark energy in the universe. From a statistical sample, it is found that the efficiency of hot gas in converting kinetic energy to thermal energy increases with time, as measured by X-ray observations within clusters. The evolution of hot gas is strongly linked to cosmic structure formation.