Near-Infrared Polarimetry of the Edge-on Galaxy NGC 891

Abstract

The edge-on galaxy NGC 891 was probed using near-infrared (NIR) imaging polarimetry in the Hband (1.6 µm) 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 • and 19 • , 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.

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

“…According to models by Wood & Jones (1997), highly polarized scattered light may be a contaminant affecting the optical and NIR polarization in edge-on systems producing polarization null points at locations along the disk, well away from the nucleus. Montgomery & Clemens (2014) do not find evidence for the predicted null points along the disk, but do find null points at other locations that they associate with an embedded spiral arm along the line of sight. Optical polarimetry (Scarrott & Draper 1996) revealed (unexpected) polarization mostly vertical to the plane, with only a few locations in the NE showing polarization parallel to the disk.…”

“…Introduction At a distance of 8.4 Mpc (Tonry et al 2001), NGC 891 presents an interesting case for an edge-on galaxy that is a late type spiral with similar mass and size compared to the Milky Way (Karachentsev et al 2004). Like the Milky Way, NIR polarimetry of NGC 891 reveals a general pattern of a magnetic field lying mostly in the plane (Jones 1997;Montgomery & Clemens 2014). Radio synchrotron observations are also consistent with this general field geometry, but extend well out of the disk into the halo (Krause 2009;Sukumar & Allen 1991).…”

“…Optical polarimetry (Scarrott & Draper 1996) revealed (unexpected) polarization mostly vertical to the plane, with only a few locations in the NE showing polarization parallel to the disk. The optical polarimetry was attributed to vertical magnetic fields, but Montgomery & Clemens (2014) argued that the optical polarimetry was contaminated by scattered light. Scattering in the halo of light from stars in the disk and the bulge, as modeled by Wood & Jones (1997) and Seon (2018), may be a more likely explanation for the optical polarization.…”

“…Over the past few decades, astronomers have detected magnetic fields in galaxies at many spatial scales. These studies have been performed using optical, NIR, CO and radio observations (see Kronberg 1994;Zweibel & Heiles 1997;Beck & Gaensler 2004;Beck 2015;Montgomery & Clemens 2014;Jones 2000;Li & Henning 2011, for example). In most nearly face-on spirals, synchrotron observations reveal a spiral pattern to the magnetic field, even in the absence of a clear spiral pattern in the surface brightness (Fletcher 2010;Beck & Gaensler 2004).…”

Probing the cold magnetised Universe with SPICA-POL (B-BOP)

“…At other wavelengths, the magnetic-field structure of a much smaller number of external galaxies has been surveyed using optical polarisation, e.g., the Magellanic Clouds (LMC and SMC; Mathewson & Ford 1970), NGC1068, and M51 (Scarrott, Ward-Thompson, & Warren-Smith 1987). Wide-field imaging of polarised extinction at near-IR wavelengths is a newer capability that has been used to probe the magnetic-field structure in nearby edge-on galaxies, with results that are generally in good agreement with radio observations (e.g., Clemens, Pavel, & Cashman 2013;Montgomery & Clemens 2014). The near-IR extinction technique is less suited to observations of face-on galaxies, due to the relatively short path length that can produce internal extinction (see, e.g., Pavel & Clemens 2012, for the case of M51).…”