Characterizing the Structure of Magnetic Fields in Spiral Galaxies with Radio and Far-Infrared Polarimetric Observations

​​Large-scale spiral magnetic field structures are commonly observed in spiral galaxies. These fields contribute to the total pressure that balances the interstellar medium against gravity as well as influence the flow of gas within a galaxy. The structure of these B-fields can be estimated through polarimetric observations and quantified as a pitch angle. Previous analyses that have characterized these pitch angles were model-dependent, e.g. assume a logarithmic spiral functional form. We propose a new method to quantify the morphology of the large-scale magnetic fields in galaxies which is more flexible and model-independent than current approaches. This method was 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 dust polarimetric observations (FIR: 154 μm) and radio (6 cm) synchrotron polarization observations. Using this new approach, we found that the average pitch angle of these large-scale B-fields was smaller and had greater angular dispersion in the FIR data compared to radio, meaning that the B-fields in the disk midplane traced by FIR dust polarization are more tightly wound and more disordered than the B-field structure in radio.