Galactic magnetic X fields

TL;DR

This paper compares a recent explanation of the galactic X magnetic field pattern with classical dynamo theory, showing how wind and rotation can generate observed magnetic structures and discussing implications for galaxy magnetic field origins.

Contribution

It demonstrates that the galactic X magnetic field pattern can arise from magnetohydrodynamic processes involving wind and rotation, challenging the necessity of a battery mechanism.

Findings

X pattern can develop from wind and rotation without a battery.

The sign distribution of Rotation Measure depends on the velocity field.

Constraints on galactic wind and magnetic field structure can be inferred from the model.

Abstract

Our aim in this note is to compare a recent explanation of the galactic X pattern in Faraday Rotation (XRM), to that produced by the advection part of the classical dynamo.We find that the characteristic X magnetic field polarization in the plane of the sky, found in edge-on spiral galaxies, can develop magnetohydromagnetically from an initial disc magnetic field combined with wind and rotation. The Rotation Measure develops a corresponding X distribution in sign, but this distribution is not a `universal' behaviour because it depends primarily on the velocity field. We use Cauchy evolution of an initial magnetic field to find the field at some later time and place. A `battery' mechanism that requires current to always flow out of a galaxy has been recently suggested, which contrasts with the conclusions of this paper. Either explanation has significant consequences for the structure of a spiral galaxy. If the battery mechanism applies then we have a new method of producing magnetic field independent of the traditional dynamo. If however the flow mechanism applies; then constraints concerning the presence and nature of a galactic wind, together with the signature of the mean radial magnetic field, can be inferred.