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

Abstract

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 magnetized media and sloshing galaxy clusters, and then applied in interpreting observations of merger cold fronts. Using a suite of magnetohydrodynamic (MHD) simulations of binary cluster mergers, we show that even magnetic field strengths stronger than yet observed (β = Pth/PB = 50) show visible KHI features. This is because our initial magnetic field is tangled, producing Alfvén waves and associated velocity fluctuations in the intracluster medium (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 that seed the KHI must be well understood and (if possible) carefully controlled.