Statistical tests of galactic dynamo theory

TL;DR

This paper compares mean-field galactic dynamo models with observational data on magnetic fields in spiral galaxies, finding reasonable agreement and highlighting parameters that can be constrained through observations.

Contribution

It provides the first systematic statistical comparison between dynamo theory predictions and galactic magnetic field observations, refining model parameters.

Findings

Dynamo models produce pitch angles consistent with observations.

Magnetic field strength estimates are within a factor of a few of observed values.

Agreement on pitch angles constrains the turbulent correlation time to 10-20 Myr.

Abstract

Mean-field galactic dynamo theory is the leading theory to explain the prevalence of regular magnetic fields in spiral galaxies, but its systematic comparison with observations is still incomplete and fragmentary. Here we compare predictions of mean-field dynamo models to observational data on magnetic pitch angle and the strength of the mean magnetic field. We demonstrate that a standard $\alpha^2\Omega$ dynamo model produces pitch angles of the regular magnetic fields of nearby galaxies that are reasonably consistent with available data. The dynamo estimates of the magnetic field strength are generally within a factor of a few of the observational values. Reasonable agreement between theoretical and observed pitch angles generally requires the turbulent correlation time $\tau$ to be in the range 10-20 Myr, in agreement with standard estimates. Moreover, good agreement also requires that the ratio of the ionized gas scale height to root-mean-square turbulent velocity increases with radius. Our results thus widen the possibilities to constrain interstellar medium (ISM) parameters using observations of magnetic fields. This work is a step toward systematic statistical tests of galactic dynamo theory. Such studies are becoming more and more feasible as larger datasets are acquired using current and up-and-coming instruments.