The Structure of Magnetic Fields in Spiral Galaxies: A Radio and Far-infrared Polarimetric Analysis

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 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.

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). VII. A Tomographic View of Far-infrared and Radio Polarimetric Observations through MHD Simulations of Galaxies

“…We will show below that these high-density regions deviating from the local inferred magnetic field often correspond, as expected, to star-forming regions. This association confirms the large angular dispersion of the FIR magnetic fields located in dense regions and the steep depolarization rate with increasing column density measured in nearby spiral galaxies (Lopez-Rodriguez et al 2022;Borlaff et al 2023;Surgent et al 2023).…”

“…Furthermore, the angular variations of the plane-of-the-sky magnetic field orientations inferred from FIR observations are larger than those measured by the radio magnetic fields (Borlaff et al 2023). Surgent et al (2023) showed that the FIR magnetic field orientations are 7%-25% more disordered than those from the magnetic fields inferred from the radio emission. Note that these variations are at the scales of the angular resolution of the observations, typically 300-500 pc.…”

“…Details regarding the radiative transfer solver for synchrotron emission used by POLARIS, the exact contribution from each cell to the Stokes parameters, and the complete computation process are provided by Reissl et al (2016Reissl et al ( , 2019. The λ = 6.2 cm wavelength (∼4.8 GHz) strikes an optimal balance of a frequently employed wavelength and the highest signal-to-noise ratio for the observed galaxies at both radio and FIR wavelengths (Lopez-Rodriguez et al 2022;Borlaff et al 2023;Surgent et al 2023). Consequently, we limit our investigation to synchrotron emission at this wavelength (i.e., 6.2 cm).…”