Magnetic Fields of Spiral Galaxies Observed with Far-Infrared Polarimetry: M 51 and NGC 891

Abstract

We explore the magnetic field geometry in two nearby spiral galaxies, M 51 and NGC 891. We look into the magnetic field geometry derived from interstellar polarization, using polarimetric images made with Stratospheric Observatory For Infrared Astronomy/High-resolution Airborne Wideband Camera-Plus at 154 μm in the far-infrared (far-IR). This study aims to characterize the magnetic fields of typical spiral galaxies in three dimensions based on the magnetic field geometry seen in the face-on galaxy, M 51, and the edge-on galaxy, NGC 891.Far-IR polarimetry in M 51 shows that the magnetic field geometry in the galactic plane is aligned with a spiral pattern delineated by gas distributions. The inferred magnetic fields in the northeastern part of the galaxy tend to diverge from the spiral pattern and point more toward the interacting companion M 51b. We derive the mean degree of polarization in M 51 based on a Bayesian theorem and the Rice probability distribution. The similar fractional polarization in the arm and inter-arm regions implies no significant difference in turbulent magnetic fields between those environments. The magnetic field geometry inferred in M 51b is not aligned with the stellar bar of the galaxy but may correspond to the orientation of a curved structure of the inner region of the bar. Magnetic fields inferred in NGC 891, an edge-on galaxy, generally lie close to the galactic plane. There is one location with vertical magnetic fields in the galactic disk, probably caused by a blowout associated with star formation. We investigate the far-IR polarimetry in NGC 891 by comparing it with simple model galaxies in which a planar, ordered magnetic field and an isotropic random field contribute to the net field geometry. The observed low fractional polarization at either side of the central region of NGC 891 is best explained as being due to the line of sight (LOS) being tangent to spiral arms, with the field aligned along the LOS. While the reduced polarization observed at the center is not well explained in the model using a normal spiral, using a barred model galaxy instead can match the observations. Overall low fractional polarization compared to other galaxies seen in the galactic plane implies significant turbulence and many turbulent decorrelation cells along the LOS through the plane. Unexpected low polarization off the galactic plane is likely due to a mixture of planar and vertical fields in the dusty regions of the halo.