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We examine the validity of the hydrostatic equilibrium (HSE) assumption forgalaxy clusters using one of the highest-resolution cosmological hydrodynamicalsimulations. We define and evaluate several effective mass terms correspondingto the Euler equations of the gas dynamics, and quantify the degree of thevalidity of HSE in terms of the mass estimate. We find that the mass estimatedunder 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 theinertial term in the Euler equations makes a negligible contribution to thetotal mass, and the overall gravity of the cluster is balanced by the thermalgas pressure gradient and the gas acceleration term. Indeed the deviation fromthe HSE mass is well explained by the acceleration term at almost all radii. Wealso clarify the confusion of previous work due to the inappropriateapplication of the Jeans equations in considering the validity of HSE from thegas dynamics extracted from cosmological hydrodynamical simulations.

Validity of Hydrostatic Equilibrium in Galaxy Clusters from Cosmological Hydrodynamical Simulations