The entropy core in galaxy clusters: numerical and physical effects in cosmological grid simulations

TL;DR

This study explores the physical and numerical origins of flat entropy cores in galaxy clusters through high-resolution simulations, revealing the dominant role of physical mixing and assessing the impact of cooling, pre-heating, and AGN feedback.

Contribution

It demonstrates that physical mixing drives entropy core formation in simulations and evaluates how different feedback mechanisms influence entropy profiles.

Findings

Entropy core mainly due to physical mixing of subsonic motions.

Numerical effects have minor influence on entropy core size.

Pre-heating and combined feedback can produce observed entropy profiles.

Abstract

We investigated the numerical and physical reasons leading to a flat distribution of low gas entropy in the core region of galaxy clusters, as commonly found in grid cosmological simulations. To this end, we run a set of 30 high resolution re-simulations of a 3 x 10^14 M_sol/h cluster of galaxies with the AMR code ENZO, exploring and investigating the details involved in the production of entropy in simulated galaxy clusters. The occurrence of the flat entropy core is found to be mainly due to hydro-dynamical processes resolved in the code and that additional spurious effects of numerical origin (e.g. artificial heating due to softening effects or N-body noise) can affect the size and level of the entropy core only in a minor way. We show that the entropy profile of non-radiative simulations is produced by a mechanism of "sorting in entropy" which takes place with regularity during the cluster evolution. Using gas tracers we prove that the flat entropy core is caused by physical mixing of gravity-driven subsonic motions within the shallow inner cluster potential. Re-simulations were also produced for the same cluster object with the addition of radiative cooling, uniform pre-heating at high redshift (z=10) and late (z<1) thermal energy feedback from AGN activity in the cluster, in order to assess the effects of such mechanisms on the final entropy profile of the cluster. We report on the infeasibility of balancing the catastrophic cooling and recovering a flat entropy profile with the investigated trials for AGN activity alone, while for a sub-set of pre-heating models, or AGN feedback plus pre-heating models, a flat entropy distribution similar to non-radiative runs can be obtained with a viable energy requirement, and in good consistency with X-ray observations.