# An azimuthally resolved study of the cold front in Abell 3667

## TL;DR

This study uses high-resolution X-ray data to analyze the cold front in galaxy cluster Abell 3667, revealing Kelvin-Helmholtz instabilities and constraining the plasma's effective viscosity to be significantly below classical estimates.

## Contribution

First observational evidence of Kelvin-Helmholtz instabilities along a galaxy cluster cold front and constraints on the plasma's effective viscosity.

## Key findings

- Detection of azimuthal variations and multiple edges indicating KHIs
- Estimated ICM viscosity is less than 5% of Spitzer viscosity
- Observed mixing suggests suppressed viscosity

## Abstract

The microphysical properties, such as effective viscosity and conductivity, of the weakly magnetized intergalactic plasma are not yet well known. We investigate the constraints that can be placed by an azimuthally resolved study of the cold front in Abell 3667 using $\sim$500 ksec archival Chandra data. We find that the radius of the interface fluctuates with position angle and the morphology of the interface is strikingly similar to recent numerical simulations of inviscid gas-stripping. We find multiple edges in the surface brightness profiles across the cold front as well as azimuthal variations, which are consistent with the presence of Kelvin-Helmholtz Instabilities (KHI) developing along the cold front. They indicate that the characteristic length scale of KHI rolls is around 20-80 kpc. This is the first observational indication of developing KHIs along a merger cold front in a galaxy cluster. Based on the KHI scenario, we estimated the upper limit of the ICM effective viscosity. The estimated value of $\mu\lesssim200~\mathrm{g/cm/s}$ is at most 5% of the isotropic Spitzer-like viscosity. The observed apparent mixing towards the outer edges away from the tip of the front provides an additional evidence for suppressed viscosity.