The Effect of Anisotropic Viscosity on Cold Fronts in Galaxy Clusters

TL;DR

This study uses high-resolution MHD simulations to investigate how anisotropic viscosity and magnetic fields influence the stability of cold fronts in galaxy clusters, revealing complex interactions that affect observable features.

Contribution

It compares the effects of isotropic and anisotropic viscosity, including magnetic fields, on cold front stability, highlighting the challenges in distinguishing suppression mechanisms from observations.

Findings

Both Braginskii and isotropic viscosity suppress K-H instabilities.

Magnetic fields significantly influence cold front appearance.

Thermal conduction rapidly erodes cold fronts regardless of viscosity.

Abstract

Cold fronts -- contact discontinuities in the intracluster medium (ICM) of galaxy clusters -- should be disrupted by Kelvin-Helmholtz (K-H) instabilities due to the associated shear velocity. However, many observed cold fronts appear stable. This opens the possibility to place constraints on microphysical mechanisms that stabilize them, such as the ICM viscosity and/or magnetic fields. We performed exploratory high-resolution simulations of cold fronts arising from subsonic gas sloshing in cluster cores using the grid-based Athena MHD code, comparing the effects of isotropic Spitzer and anisotropic Braginskii viscosity (expected in a magnetized plasma). Magnetized simulations with full Braginskii viscosity or isotropic Spitzer viscosity reduced by a factor f ~ 0.1 are both in qualitative agreement with observations in terms of suppressing K-H instabilities. The RMS velocity of turbulence within the sloshing region is only modestly reduced by Braginskii viscosity. We also performed unmagnetized simulations with and without viscosity and find that magnetic fields have a substantial effect on the appearance of the cold fronts, even if the initial field is weak and the viscosity is the same. This suggests that determining the dominant suppression mechanism of a given cold front from X-ray observations (e.g. viscosity or magnetic fields) by comparison with simulations is not straightforward. Finally, we performed simulations including anisotropic thermal conduction, and find that including Braginskii viscosity in these simulations does not significant affect the evolution of cold fronts; they are rapidly smeared out by thermal conduction, as in the inviscid case.