Steady Galactic Dynamos and Observational Consequences I: Halo Magnetic Fields

TL;DR

This paper investigates how steady, axially symmetric galactic dynamos influence halo magnetic fields, predicting observable sign changes in Faraday rotation measures, supported by CHANG-ES survey observations.

Contribution

It introduces a scale-invariant dynamo model incorporating velocity fields as parameters, predicting halo magnetic field configurations and rotation measure oscillations.

Findings

Predicted off-axis, spirally wound magnetic fields in galaxy halos.

Sign changes in Faraday rotation measures across galaxy quadrants.

Observed rotation measure oscillations in CHANG-ES survey data.

Abstract

We study the global consequences in the halos of spiral galaxies of the steady, axially symmetric, mean field dynamo. We use the classical theory but add the possibility of using the velocity field components as parameters in addition to the helicity and diffusivity. The analysis is based on the simplest version of the theory and uses scale-invariant solutions. The velocity field (subject to restrictions) is a scale invariant field in a `pattern' frame, in place of a full dynamical theory. The `pattern frame' of reference may either be the systemic frame or some rigidly rotating spiral pattern frame. One type of solution for the magnetic field yields off-axis, spirally wound, magnetic field lines. These predict sign changes in the Faraday screen rotation measure in every quadrant of the halo of an edge-on galaxy. Such rotation measure oscillations have been observed in the CHANG-ES survey.