Near-infrared polarimetry of a normal spiral galaxy viewed through the Taurus Molecular Cloud Complex

TL;DR

This study used near-infrared polarimetry to map magnetic fields in a spiral galaxy seen through the Taurus Molecular Cloud, revealing disk-parallel magnetic field structures consistent with radio observations.

Contribution

First near-infrared polarimetric mapping of a normal spiral galaxy through a molecular cloud, demonstrating magnetic field coherence across different interstellar media.

Findings

Galaxy exhibits disk-parallel magnetic fields.

Foreground polarization was accurately measured and removed.

Infrared polarization aligns with radio synchrotron results.

Abstract

Few normal galaxies have been probed using near-infrared polarimetry, even though it reveals magnetic fields in the cool interstellar medium better than either optical or radio polarimetry. Deep H-band (1.6um) linear imaging polarimetry toward Taurus serendipitously included the galaxy 2MASX J04412715+2433110 with adequate sensitivity and resolution to map polarization across nearly its full extent. The observations revealed the galaxy to be a steeply inclined (~75 deg) disk type with a diameter, encompassing 90% of the Petrosian flux, of 4.2 kpc at a distance of 53 Mpc. Because the sight line passes through the Taurus Molecular Cloud complex, the foreground polarization needed to be measured and removed. The foreground extinction Av of 2.00+/-0.10 mag and reddening E(H-K) of 0.125 +/- 0.009 mag were also assessed and removed, based on analysis of 2MASS, UKIDSS, Spitzer, and WISE photometry using the NICE, NICER, and RJCE methods. Corrected for the polarized foreground, the galaxy polarization values range from zero to 3%. The polarizations are dominated by a disk-parallel magnetic field geometry, especially to the northeast, while either a vertical field or single scattering of bulge light produces disk-normal polarizations to the southwest. The multi-kpc coherence of the magnetic field revealed by the infrared polarimetry is in close agreement with short wavelength radio synchrotron observations of edge-on galaxies, indicating that both cool and warm interstellar media of disk galaxies may be threaded by common magnetic fields.