| Abstract: Magnetic fields play a crucial role in maintaining the structure and dynamics of galactic environments. On kiloparsec scales, galactic magnetic fields are ordered and consist of two components: a large-scale regular (mean) field and an anisotropic random field. Although magnetic fields cannot be measured directly, the ordered component produces polarized radio emission. By using multi-frequency radio polarization observations (Stokes Q and U parameters) between 3 cm and 20 cm from the Very Large Array (VLA) and the Effelsberg telescope, we can probe the magnetic field structure in the galactic plane through the polarization angle.
The observed polarization angle has two contributions: an intrinsic component, which traces the orientation of the ordered magnetic field in the plane of the galaxy, and a Faraday rotation component, which depends on the line-of-sight component of the regular magnetic field. Assuming the contribution from the anisotropic random field to be negligible, we model the large-scale magnetic field using a Fourier decomposition in multiple concentric rings, spanning 6–14 kpc for M31 and 2.5–17.5 kpc for IC 342.
Our analysis shows that the magnetic fields in both M31 and IC 342 are dominated by the axisymmetric (m = 0) mode. However, higher-order modes are also present, capturing localized variations in the field structure (m = 1, 2, 3 for M31 and m = 1 for IC 342). Notably, the modeled magnetic field successfully reproduces not only the intrinsic polarization angles but also the Faraday rotation component. This result suggests either that the anisotropic random field is significantly weaker than the regular field, contrary to earlier findings (e.g., Beck et al. 2019), or it is preferentially aligned with the regular magnetic field. These findings place strong new constraints on the role of anisotropic turbulence in spiral galaxies and provide critical observational input for refining large-scale galactic dynamo models. |
