The fine structure of the mean magnetic field in M31

TL;DR

This study analyzes the spatial structure of M31's magnetic field using Fourier and Bayesian methods, revealing dominant axisymmetric and sub-dominant modes, with implications for galactic dynamo theory.

Contribution

It introduces a Bayesian inference approach to analyze magnetic field modes in M31, accounting for depolarization effects and revealing complex azimuthal variations.

Findings

Dominant axisymmetric magnetic mode aligns with dynamo theory.

Higher modes show irregular variation with galactocentric radius.

Magnetic field strength peaks at 10-14 kpc, varying azimuthally.

Abstract

To explore the spatial variations of the regular (mean) magnetic field of the Andromeda galaxy (M31), we use Fourier analysis in azimuthal angle along four rings in the galaxy's plane. The Fourier coefficients give a quantitative measure of strength of the modes, enabling us to compare expectations from mean-field dynamo models of spiral galaxies. Earlier analyses indicated that the axisymmetric magnetic field (azimuthal Fourier mode $m=0$) is sufficient to fit the observed polarization angles in a wide range of galactocentric distances ($r$). We apply a Bayesian inference approach to new, more sensitive radio continuum data at $\lambda \lambda3.59$, $6.18$, and $11.33$ cm and the earlier data at $\lambda 20.46$ cm to reveal sub-dominant contributions from the modes $m=1$, 2, and 3 along with a dominant axisymmetric mode. Magnetic lines of the axisymmetric mode are close to trailing logarithmic spirals which are significantly more open than the spiral arms detectable in the interstellar dust and neutral hydrogen. The form of the $m=0$ mode is consistent with galactic dynamo theory. Both the amplitudes and the pitch angles of the higher azimuthal modes ($m>1$) vary irregularly with $r$ reflecting local variations in the magnetic field structure. The maximum strength of the mean magnetic field of $1.8-2.7 \mu$G (for the axisymmetric part of the field) occurs at $10-14$ kpc but we find that its strength varies strongly along the azimuth; this variation gives rise to the $m=1$ mode. We suggest a procedure of Bayesian inference which is independent of the specific nature of the depolarization and applies when the magneto-ionic layer observable in polarized emission is not symmetric along the line of sight because emission from its far side is completely depolarized.