Validity of Hydrostatic Equilibrium in Galaxy Clusters from Cosmological Hydrodynamical Simulations

TL;DR

This study assesses the accuracy of the hydrostatic equilibrium assumption in galaxy clusters using high-resolution simulations, finding deviations up to 30% primarily due to gas acceleration effects rather than inertial terms.

Contribution

It provides a detailed analysis of the factors affecting hydrostatic mass estimates, clarifying the role of gas acceleration and correcting previous misconceptions about inertial contributions.

Findings

HSE mass deviates from true mass by up to ~30%.

Gas acceleration explains most deviations from HSE.

Inertial terms are negligible in the mass balance.

Abstract

We examine the validity of the hydrostatic equilibrium (HSE) assumption for galaxy clusters using one of the highest-resolution cosmological hydrodynamical simulations. We define and evaluate several effective mass terms corresponding to the Euler equations of the gas dynamics, and quantify the degree of the validity of HSE in terms of the mass estimate. We find that the mass estimated under the HSE assumption (the HSE mass) deviates from the true mass by up to ~ 30 %. This level of departure from HSE is consistent with the previous claims, but our physical interpretation is rather different. We demonstrate that the inertial term in the Euler equations makes a negligible contribution to the total mass, and the overall gravity of the cluster is balanced by the thermal gas pressure gradient and the gas acceleration term. Indeed the deviation from the HSE mass is well explained by the acceleration term at almost all radii. We also clarify the confusion of previous work due to the inappropriate application of the Jeans equations in considering the validity of HSE from the gas dynamics extracted from cosmological hydrodynamical simulations.