The structure of magnetic fields in spiral galaxies: a radio and far-infrared polarimetric analysis

TL;DR

This paper introduces a novel, flexible method to analyze the large-scale magnetic field structures in spiral galaxies using polarimetric data from radio and far-infrared observations, revealing differences in magnetic field morphology.

Contribution

It adapts a polarization analysis technique from EHT data to galaxy magnetic fields, providing a new, model-independent way to quantify magnetic field modes and structures.

Findings

Main magnetic modes are m=2 and m=0 at FIR, and m=2 at radio wavelengths.

FIR data shows more contribution from various modes than radio data.

Magnetic pitch angles are smaller and more chaotic in FIR than in radio observations.

Abstract

We propose and apply a method to quantify the morphology of the large-scale ordered magnetic fields (B-fields) in galaxies. This method is adapted from the analysis of Event Horizon Telescope polarization data. We compute a linear decomposition of the azimuthal modes of the polarization field in radial galactocentric bins. We apply this approach to five low-inclination spiral galaxies with both far-infrared (FIR: 154 $\mu$m) dust polarimetric observations taken from the Survey of ExtragALactic magnetiSm with SOFIA (SALSA) and radio (6 cm) synchrotron polarization observations. We find that the main contribution to the B-field structure of these spiral galaxies comes from the $m=2$ and $m=0$ modes at FIR wavelengths and the $m=2$ mode at radio wavelengths. The $m=2$ mode has a spiral structure and is directly related to the magnetic pitch angle, while $m=0$ has a constant B-field orientation. The FIR data tend to have a higher relative contribution from other modes than the radio data. The extreme case is NGC 6946: all modes contribute similarly in the FIR, while $m=2$ still dominates in the radio. The average magnetic pitch angle in the FIR data is smaller and has greater angular dispersion than in the radio, indicating that the B-fields in the disk midplane traced by FIR dust polarization are more tightly wound and more chaotic than the B-field structure in the radio, which probes a larger volume. We argue that our approach is more flexible and model-independent than standard techniques, while still producing consistent results where directly comparable.