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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.