Detecting the orientation of magnetic fields in galaxy clusters

TL;DR

This paper uses 3D magnetohydrodynamical simulations to explain observed magnetic field structures in galaxy clusters and introduces a new method to map and analyze these magnetic fields, revealing radial orientations consistent with theoretical predictions.

Contribution

It presents a novel technique for probing cluster magnetic fields through polarized emission from galaxies, supported by simulation-based explanations of observed magnetic ridges.

Findings

Magnetic ridges are explained by galaxies sweeping up field lines.

The technique maps magnetic field orientations in galaxy clusters.

Field in Virgo cluster is predominantly radial outside the core.

Abstract

Clusters of galaxies, filled with hot magnetized plasma, are the largest bound objects in existence and an important touchstone in understanding the formation of structures in our Universe. In such clusters, thermal conduction follows field lines, so magnetic fields strongly shape the cluster's thermal history; that some have not since cooled and collapsed is a mystery. In a seemingly unrelated puzzle, recent observations of Virgo cluster spiral galaxies imply ridges of strong, coherent magnetic fields offset from their centre. Here we demonstrate, using three-dimensional magnetohydrodynamical simulations, that such ridges are easily explained by galaxies sweeping up field lines as they orbit inside the cluster. This magnetic drape is then lit up with cosmic rays from the galaxies' stars, generating coherent polarized emission at the galaxies' leading edges. This immediately presents a technique for probing local orientations and characteristic length scales of cluster magnetic fields. The first application of this technique, mapping the field of the Virgo cluster, gives a startling result: outside a central region, the magnetic field is preferentially oriented radially as predicted by the magnetothermal instability. Our results strongly suggest a mechanism for maintaining some clusters in a 'non-cooling-core' state.