HAWC+ Far-Infrared Observations of the Magnetic Field Geometry in M51 and NGC 891.

SOFIA HAWC+ polarimetry at 154 m is reported for the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors (1 􀀀 9%, rotated to show the inferred magnetic eld direction) generally follow the spiral pattern dened by the molecular gas distribution, the far- infrared (FIR) intensity contours, and other tracers of star formation. Accounting for observational errors and the uncertain contribution of a random component to the magnetic eld, the FIR vectors in the outer arms may be consistent with the spiral pattern, but in the center of M51 the vectors show a more open pattern than the tracers. The fractional polarization is much lower in the FIR- Corresponding author: Terry Jay Jones [e-mail] 2 Jones et al. bright central regions than in the outer regions, and we rule out loss of grain alignment and variations in magnetic eld strength as causes. When compared with existing synchrotron observations, which sample dierent regions with dierent weighting, we nd the net position angles are strongly correlated, the fractional polarizations are moderately correlated, but the polarized intensities are uncorrelated. We argue that the low fractional polarization in the central regions must be due to signicant numbers of highly turbulent segments across the beam and along lines of sight in the beam in the central 3 kpc of M51. For the region further out from the center, the fractional polarization is statistically the same in the relatively poorly sampled inter-arm regions and in the spiral arms themselves, indicating the random component to the eld geometry in our beam is similar between these regions. For NGC 891, the FIR polarization vectors within an intensity contour of 1500 MJy sr􀀀1 are oriented very close to the plane of the galaxy, unlike the near-infrared (NIR) and radio measurements of the magnetic eld geometry, which show a clear 15 oset from the plane at many locations. The FIR polarimetry is probably sampling the magnetic eld geometry in NGC 891 much deeper into the disk than is possible with NIR polarimetry and radio synchrotron measurements. In some locations in NGC 891 the FIR polarization is very low, suggesting we are preferentially viewing the magnetic eld mostly along the line of sight, down the length of embedded spiral arms. There is tentative evidence for a vertical eld in the polarized emission o the plane of the disk, possibly associated with vertical dust columns seen at optical wavelengths.