Near-Infrared Polarimetry of the Edge-On Galaxy NGC891
  The edge-on galaxy NGC 891 was probed using near-infrared (NIR) imaging
polarimetry in the H-band (1.6 um) with the Mimir instrument on the 1.8 m
Perkins Telescope. Polarization was detected with signal-to-noise ratio greater
than three out to a surface brightness of 18.8 mag arcsec^-2. The unweighted
average and dispersion in polarization percentage (P) across the full disk were
0.7% and 0.3%, respectively, and the same quantities for polarization position
angle (P.A.) were 12 deg and 19 deg, respectively. At least one polarization
null point, where P falls nearly to zero, was detected in the NE disk but not
the SW disk. Several other asymmetries in P between the northern and southern
disk were found and may be related to spiral structure. Profiles of P and P.A.
along the minor axis of NGC 891 suggest a transition from magnetic (B) field
tracing dichroic polarization near the disk mid-plane to scattering dominated
polarization off the disk mid-plane. A comparison between NIR P.A. and radio
(3.6 cm) synchrotron polarization P.A. values revealed similar B-field
orientations in the central-northeast region, which suggests that the hot
plasma and cold, star-forming interstellar medium may share a common B-field.
Disk-perpendicular polarizations previously seen at optical wavelengths are
likely caused by scattered light from the bright galaxy center and are unlikely
to be tracing poloidal B-fields in the outer disk.
