The Effect of Anisotropic Viscosity on Cold Fronts in Galaxy Clusters
  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.
