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Large-scale spiral magnetic field structures are commonly observed in spiral galaxies. These magnetic fields can contribute to the total pressure that balances the interstellar medium against gravity to form stars as well as influence the flow of gas within a galaxy. The structure of these magnetic fields can be quantified through polarimetric observations by estimating the pitch angle. Previous analyses that have characterized these pitch angles were model-dependent, i.e. assuming logarithmic spirals and then fitting them to the spiral arms of the gas tracers. In this talk, I will present 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. We adapt this method from the analysis of Event Horizon Telescope polarization data. We apply this method to five low-inclination spiral galaxies with both far-infrared (FIR: 154 μm) dust polarimetric observations and radio (6 cm) synchrotron polarization observations. The magnetic field orientation at FIR and radio wavelengths are different. We compute the pitch angles from FIR and radio wavelengths as a function of galactocentric radius. We find that the main contribution to the magnetic 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. Additionally, the FIR magnetic fields seem to be more disordered than radio magnetic fields. This suggests that the FIR magnetic fields, mainly located in the spiral arms, may be more sensitive to the activity of the star-forming regions than the radio magnetic fields, which are mainly located in the interarms.
Note: Will is actually an advanced undergraduate at Stanford, and he comes highly recommended by his colleagues. As such, we've allowed him to make a somewhat shorter presentation than we're used to here. Looking forward to it!