2023/02/14 by William Jeffrey Surgent, Enrique López-Rodríguez, Surgent, William Jeffrey +3 · 1 citation
Physics and Astronomy · #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Pulsars and Gravitational Waves Research #Stellar, planetary, and galactic studies
paper · pdf · doi:10.48550/arxiv.2302.07278
openalex publication_date 2023/02/14 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
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 μm) 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.