Anisotropic Thermal Conduction and the Cooling Flow Problem in Galaxy Clusters

TL;DR

This study uses 3D MHD simulations to investigate the impact of anisotropic thermal conduction and the heat-flux-driven buoyancy instability on the cooling flow problem in galaxy clusters, showing conduction's limited stabilizing effect.

Contribution

It demonstrates that the heat-flux-driven buoyancy instability significantly suppresses thermal conduction in cluster cores, challenging the effectiveness of conduction alone in preventing cooling catastrophes.

Findings

HBI reduces effective radial thermal conductivity to less than 10% of Spitzer value.

Conduction alone cannot prevent cooling catastrophes in low-entropy cluster cores.

High-entropy clusters can benefit from conduction to delay cooling issues.

Abstract

We examine the long-standing cooling flow problem in galaxy clusters with 3D MHD simulations of isolated clusters including radiative cooling and anisotropic thermal conduction along magnetic field lines. The central regions of the intracluster medium (ICM) can have cooling timescales of ~200 Myr or shorter--in order to prevent a cooling catastrophe the ICM must be heated by some mechanism such as AGN feedback or thermal conduction from the thermal reservoir at large radii. The cores of galaxy clusters are linearly unstable to the heat-flux-driven buoyancy instability (HBI), which significantly changes the thermodynamics of the cluster core. The HBI is a convective, buoyancy-driven instability that rearranges the magnetic field to be preferentially perpendicular to the temperature gradient. For a wide range of parameters, our simulations demonstrate that in the presence of the HBI, the effective radial thermal conductivity is reduced to less than 10% of the full Spitzer conductivity. With this suppression of conductive heating, the cooling catastrophe occurs on a timescale comparable to the central cooling time of the cluster. Thermal conduction alone is thus unlikely to stabilize clusters with low central entropies and short central cooling timescales. High central entropy clusters have sufficiently long cooling times that conduction can help stave off the cooling catastrophe for cosmologically interesting timescales.