FIELDMAPS Data Release: Far-Infrared Polarization in the "Bones" of the Milky Way

Polarization observations of the Milky Way and many other spiral galaxies have found a close correspondence between the orientation of spiral arms and magnetic field lines on scales of hundreds of parsecs. This paper presents polarization measurements at 214 $\mu$m toward ten filamentary candidate ``bones" in the Milky Way using the High-resolution Airborne Wide-band Camera (HAWC+) on the Stratospheric Observatory for Infrared Astronomy (SOFIA). These data were taken as part of the Filaments Extremely Long and Dark: A Magnetic Polarization Survey (FIELDMAPS) and represent the first study to resolve the magnetic field in spiral arms at parsec scales. We describe the complex yet well-defined polarization structure of all ten candidate bones, and we find a mean difference and standard deviation of $-74^{\circ} \pm 32^{\circ}$ between their filament axis and the plane-of-sky magnetic field, closer to a field perpendicular to their length rather than parallel. By contrast, the 850 $\mu$m polarization data from \textit{Planck} on scales greater than 10 pc show a nearly parallel mean difference of $3^{\circ} \pm 21^{\circ}$. These findings provide further evidence that magnetic fields can change orientation at the scale of dense molecular clouds, even along spiral arms. Finally, we use a power law to fit the dust polarization fraction as a function of total intensity on a cloud-by-cloud basis and find indices between $-0.6$ and $-0.9$, with a mean and standard deviation of $-0.7 \pm 0.1$. The polarization, dust temperature, and column density data presented in this work are publicly available online.