Irregular sloshing cold fronts in the nearby merging groups NGC 7618 and UGC 12491: evidence for Kelvin-Helmholtz instabilities

TL;DR

This study uses Chandra X-ray observations to reveal distorted cold fronts in galaxy groups, providing evidence for Kelvin-Helmholtz instabilities and suggesting variations in magnetic fields and viscosity in the intracluster medium.

Contribution

First detection of KHI-induced distortions in cold fronts of galaxy groups, challenging previous assumptions about magnetic suppression effects.

Findings

Cold fronts exhibit 'wings' indicating KHIs.

Distorted fronts contrast with smooth ones in other systems.

Implications for magnetic fields and viscosity in the ICM.

Abstract

We present results from two \sim30 ks Chandra observations of the hot atmospheres of the merging galaxy groups centered around NGC 7618 and UGC 12491. Our images show the presence of arc-like sloshing cold fronts wrapped around each group center and \sim100 kpc long spiral tails in both groups. Most interestingly, the cold fronts are highly distorted in both groups, exhibiting 'wings' along the fronts. These features resemble the structures predicted from non-viscous hydrodynamic simulations of gas sloshing, where Kelvin-Helmholtz instabilities (KHIs) distort the cold fronts. This is in contrast to the structure seen in many other sloshing and merger cold fronts, which are smooth and featureless at the current observational resolution. Both magnetic fields and viscosity have been invoked to explain the absence of KHIs in these smooth cold fronts, but the NGC 7618/UGC 12491 pair are two in a growing number of both sloshing and merger cold fronts that appear distorted. Magnetic fields and/or viscosity may be able to suppress the growth of KHIs at the cold fronts in some clusters and groups, but clearly not in all. We propose that the presence or absence of KHI-distortions in cold fronts can be used as a measure of the effective viscosity and/or magnetic field strengths in the ICM.