The structure of magnetic fields in spiral galaxies: a radio and far-infrared polarimetric analysis

Kavli Affiliate: Susan E. Clark

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| Summary:

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: $154~mu$m ) 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 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
$m=2$ mode has a spiral structure and is directly related to the magnetic pitch
angle, while $m=0$ has a constant B-field orientation. The 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.

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