Effects of Viscosity on Sloshing Cold Fronts in Galaxy Clusters

TL;DR

This study investigates how different viscosity models and magnetic field strengths influence the stability of cold fronts in galaxy clusters, revealing that viscosity and magnetic fields significantly suppress instabilities like Kelvin-Helmholtz, affecting observable features.

Contribution

It compares multiple ICM viscosity models and magnetic field strengths in simulations to determine their effects on cold front stability in galaxy clusters.

Findings

Isotropic Spitzer viscosity suppresses KHI, producing smooth fronts.

Inviscid and microinstability-limited cases show prominent ripples.

Stronger magnetic fields further suppress KHI and smoothen fronts.

Abstract

The viscous properties of the intracluster medium (ICM) remain poorly constrained. Cold fronts-sharp discontinuities formed during cluster mergers-offer a potential avenue to probe the effective viscosity of the ICM. Velocity shear across these fronts should generate Kelvin-Helmholtz instabilities (KHI), unless viscosity or magnetic tension suppresses them. We perform cluster merger simulations incorporating four ICM viscosity models: (A) inviscid, (B) isotropic Spitzer viscosity, (C) anisotropic Braginskii viscosity, and (D) Braginskii viscosity limited by microinstabilities. The isotropic Spitzer viscosity (case B) strongly suppresses KHI, producing smooth cold front surfaces, while the inviscid (A) and microinstability-limited (D) cases show prominent ripples. The Braginskii case (C) yields intermediate suppression. We also vary the plasma $\beta$ parameter ($\beta \approx$ 100 and 1600) to examine how a changing magnetic field strength affects the results. Stronger magnetic fields further suppress KHI, leading to smoother fronts and reduced differences between different viscosity models, while also widening the range of permitted pressure anisotropies when microinstability-based limiters are present. These results indicate that both viscosity and magnetic fields play crucial roles in stabilising sloshing cold fronts in galaxy clusters.