HAWC+ Far-Infrared Observations of the Magnetic Field Geometry in M51
  and NGC 891
  SOFIA HAWC+ polarimetry at $154~\micron$ is reported for the face-on galaxy
M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors generally
follow the spiral pattern defined by the molecular gas distribution, the
far-infrared (FIR) intensity contours, and other tracers of star formation. The
fractional polarization is much lower in the FIR-bright central regions than in
the outer regions, and we rule out loss of grain alignment and variations in
magnetic field strength as causes. When compared with existing synchrotron
observations, which sample different regions with different weighting, we find
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 significant numbers of highly turbulent segments across the beam and along
lines of sight in the beam in the central 3 kpc of M51. For NGC 891, the FIR
polarization vectors within an intensity contour of 1500 $\rm{MJy~sr^{-1}}$ are
oriented very close to the plane of the galaxy. The FIR polarimetry is probably
sampling the magnetic field 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 field mostly along the line of sight, down
the length of embedded spiral arms. There is tentative evidence for a vertical
field in the polarized emission off the plane of the disk.
