Galaxy Motions, Turbulence and Conduction in Clusters of Galaxies

TL;DR

This study uses 3D MHD simulations to explore how turbulence and thermal conduction in galaxy clusters influence cooling processes, revealing that turbulence enhances conduction, which can prevent catastrophic cooling and impact cluster evolution.

Contribution

The paper demonstrates that turbulence and thermal conduction work together to regulate cooling in galaxy clusters, with detailed simulations showing their interplay and effects on magnetic fields and gas dynamics.

Findings

Turbulence is volume-filling and excites large-scale g-modes.

Thermal conduction and turbulence together prevent or delay cooling catastrophes.

Magnetic fields are amplified by turbulence, correlating with gas vorticity growth.

Abstract

Unopposed radiative cooling in clusters of galaxies results in excessive mass deposition rates. 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 center. These processes can either reduce the energy requirements for AGN heating of cool cores, or they can prevent overcooling altogether. We perform 3D MHD 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. For a wide range of observationally motivated galaxy parameters, 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 conditions in the intracluster medium. Whilst dissipation of turbulent motions is negligible as a heat source, turbulent heat diffusion is extremely important; it predominates in the cluster center. 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.