Evolution of galactic magnetic fields

TL;DR

This study models the evolution of magnetic fields in spiral galaxies over cosmic time using a semi-analytic approach combined with dynamo theory, revealing when large-scale fields develop and matching observed pitch angles.

Contribution

It introduces a combined semi-analytic and dynamo modeling framework to predict the evolution of galactic magnetic fields across cosmic history.

Findings

Most massive galaxies have large-scale magnetic fields at present.

Large-scale dynamo activity typically starts at redshift z<3.

Radial profiles of magnetic pitch angles agree with observations.

Abstract

We study the cosmic evolution of the magnetic fields of a large sample of spiral galaxies in a cosmologically representative volume by employing a semi-analytic galaxy formation model and numerical dynamo solver in tandem. We start by deriving time- and radius-dependent galaxy properties using the galform galaxy formation model, which are then fed into the nonlinear mean-field dynamo equations. These are solved to give the large-scale (mean) field as a function of time and galactocentric radius for a thin disc, assuming axial symmetry. A simple prescription for the evolution of the small-scale (random) magnetic field component is also adopted. We find that, while most massive galaxies are predicted to have large-scale magnetic fields at redshift z=0, a significant fraction of them are expected to contain negligible large-scale field. Our model indicates that, for most of the galaxies containing large-scale magnetic fields today, the mean-field dynamo becomes active at z<3. We compute the radial profiles of pitch angle, and find broad agreement with observational data for nearby galaxies.