Turbulent magnetic fields in merging clusters: A case study of Abell 2146

TL;DR

This study uses MHD simulations to show that tangled magnetic fields in merging galaxy clusters can produce observable Kelvin-Helmholtz instabilities, complicating the inference of magnetic field strength from observations.

Contribution

The paper demonstrates that tangled magnetic fields can generate KHI features even at high plasma beta, challenging previous assumptions about magnetic tension effects.

Findings

Stronger initial magnetic fields lead to more visible KHI features.

Turbulent perturbations can mimic KHI signatures, complicating magnetic field measurements.

Magnetic field tangling influences surface brightness and temperature fluctuations.

Abstract

Kelvin-Helmholtz Instabilities (KHI) along contact discontinuities in galaxy clusters have been used to constrain the strength of magnetic fields in galaxy clusters, following the assumption that, as magnetic field lines drape around the interface between the cold and hot phases, their magnetic tension resists the growth of perturbations. This has been observed in simulations of rigid objects moving through magnetised media and sloshing galaxy clusters, and then applied in interpreting observations of merger cold fronts. Using a suite of MHD simulations of binary cluster mergers, we show that even magnetic field strengths stronger than yet observed ($\beta = P_{\rm th}/P_B = 50$) show visible KHI features. This is because our initial magnetic field is tangled, producing Alfven waves and associated velocity fluctuations in the ICM; stronger initial fields therefore seed larger fluctuations, so that even a reduced growth rate due to magnetic tension produces a significant KHI. The net result is that a stronger initial magnetic field produces more dramatic fluctuations in surface brightness and temperature, not the other way around. We show that this is hard to distinguish from the evolution of turbulent perturbations of the same initial magnitude. Therefore, in order to use observations of KHI in the ICM to infer magnetic field strengths by comparing to idealized simulations, the perturbations which seed the KHI must be well-understood and (if possible) carefully controlled.