Galaxy motions, turbulence and conduction in clusters of galaxies

Abstract

Unopposed radiative cooling in clusters of galaxies results in excessive mass deposition rates on to the central brightest cluster galaxy. However, the cool cores of galaxy clusters are continuously heated by thermal conduction and turbulent heat diffusion due to minor mergers or the galaxies orbiting the cluster centre. These processes can either reduce the energy requirements for active galactic nucleus heating of cool cores, or they can prevent overcooling altogether. We perform three‐dimensional magnetohydrodynamics simulations including field‐aligned thermal conduction and self‐gravitating particles to model this in detail. Turbulence is not confined to the wakes of galaxies but is instead volume filling, due to the excitation of large‐scale g‐modes. We systematically probe the parameter space of galaxy masses and numbers to assess when the cooling catastrophe is prevented. For a wide range of observationally motivated galaxy parameters, we find that the magnetic field is randomized by stirring motions, restoring the conductive heat flow to the core. The cooling catastrophe either does not occur or it is sufficiently delayed to allow the cluster to experience a major merger that could reset the conditions in the intracluster medium. Whilst dissipation of turbulent motions (and hence dynamical friction heating) is negligible as a heat source, turbulent heat diffusion is extremely important; it predominates in the cluster centre. However, thermal conduction becomes important at larger radii, and simulations without thermal conduction suffer a cooling catastrophe. Conduction is important both as a heat source and to reduce stabilizing buoyancy forces, enabling more efficient diffusion. Turbulence enables conduction, and conduction enables turbulence. In these simulations, the gas vorticity – which is a good indicator of trapped g‐modes – increases with time. The vorticity growth is approximately mirrored by the growth of the magnetic field, which is amplified by turbulence.