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Lookup NU author(s): Professor Anvar ShukurovORCiD
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
© The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society.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. We apply a Bayesian inference approach to new, more sensitive radio continuum data at λλ3.59, 6.18, and 11.33 cm and the earlier data at λ20.46 cm to reveal subdominant 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 μG (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.
Author(s): Paul I, Kashyap RV, Ghosh T, Beck R, Chamandy L, Sinha S, Shukurov A
Publication type: Article
Publication status: Published
Journal: Monthly Notices of the Royal Astronomical Society
Year: 2026
Volume: 545
Issue: 2
Online publication date: 21/11/2025
Acceptance date: 07/11/2025
Date deposited: 06/01/2026
ISSN (print): 0035-8711
ISSN (electronic): 1365-2966
Publisher: Oxford University Press
URL: https://doi.org/10.1093/mnras/staf2041
DOI: 10.1093/mnras/staf2041
Data Access Statement: The software used in this work is publicly available: ASTROPY (Robitaille, T.P. and Astropy Collaboration 2013; Price-Whelan, A. M. and Astropy Collaboration 2018), MATPLOTLIB (J. D. Hunter 2007), COBAYA (J. Torrado & A. Lewis 2021), and GETDIST (A. Lewis 2019). We use the publicly available M31 data (https://cdsarc .cds.unistra.fr/ftp/J/A+A/633/A5/fits/) for the analysis of this work
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