Galactic magnetic fields II. Applying the model to nearby galaxies

TL;DR

This paper applies a simple galactic dynamo model to nearby galaxies, comparing theoretical predictions with observations, and finds that additional effects like spiral arms are needed to fully explain magnetic field pitch angles.

Contribution

It extends a previous simple dynamo model to multiple galaxies and compares its predictions with observed magnetic field properties, highlighting the need for additional physical effects.

Findings

Model outputs agree fairly well with observed galaxy properties.

Most parameter values are similar across different galaxies.

Predicted magnetic pitch angles are smaller than observed, indicating missing effects.

Abstract

Many spiral galaxies host magnetic fields with energy densities comparable to those of the turbulent and thermal motions of their interstellar gas. However, quantitative comparison between magnetic field properties inferred from observation and those obtained from theoretical modeling has been lacking. In Paper I we developed a simple, axisymmetric galactic dynamo model that uses various observational data as input. Here we apply our model to calculate radial profiles of azimuthally and vertically averaged magnetic field strength and pitch angle, gas velocity dispersion and scale height, turbulent correlation time and length, and the sizes of supernova remnants for the galaxies M31, M33, M51, and NGC 6946, using input data collected from the literature. Scaling factors are introduced to account for a lack of precision in both theory and observation. Despite the simplicity of our model, its outputs agree fairly well with galaxy properties inferred from observation. Additionally, we find that most of the parameter values are similar between galaxies. We extend the model to predict the magnetic field pitch angles arising from a combination of mean-field dynamo action and the winding up of the random small-scale field owing to the large-scale radial shear. We find their magnitudes to be much smaller than those of the pitch angles measured in polarized radio and far infrared emission. This suggests that effects not included in our model, such as effects associated with spiral arms, are needed to explain the pitch angle values.