Using Faraday Rotation to Probe MHD Instabilities in Intracluster Media

TL;DR

This paper proposes using Faraday rotation measurements to detect and distinguish conduction-driven MHD instabilities in intracluster media, which influence magnetic field structures and can be observed with future radio telescopes.

Contribution

The study develops 3D models of magnetized cooling core clusters and demonstrates that high-sensitivity RM observations can differentiate between magnetic field configurations caused by MHD instabilities and turbulence.

Findings

RM maps can distinguish magnetic field geometries

Future radio telescopes can detect MHD instabilities

Magnetic topology reveals underlying physical processes

Abstract

It has recently been suggested that conduction-driven magnetohydrodynamic (MHD) instabilities may operate at all radii within an intracluster medium (ICM), and profoundly affect the structure of a cluster's magnetic field. Where MHD instabilities dominate the dynamics of an ICM, they will re-orient magnetic field lines perpendicular to the temperature gradient inside a cooling core, or parallel to the temperature gradient outside it. This characteristic structure of magnetic field could be probed by measurements of polarized radio emission from background sources. Motivated by this possibility we have constructed 3-d models of a magnetized cooling core cluster and calculated Faraday rotation measure (RM) maps in the plane of the sky under realistic observing conditions. We compare a scenario in which magnetic field geometry is characterized by conduction driven MHD instabilities to that where it is determined by isotropic turbulent motions. We find that future high-sensitivity spectro-polarimetric measurements of RM, such as will be enabled by the Expanded Very Large Array and Square Kilometer Array can distinguish between these two cases with plausible exposure times. Such observations will test the existence of conduction-driven MHD instabilities in dynamically relaxed cooling core clusters. More generally, our findings imply that observations of Faraday RM should be able to discern physical mechanisms that result in qualitatively different magnetic field topologies, without a priori knowledge about the nature of the processes.