First Results from the Submillimeter Polarimeter for Antarctic Remote Observations: Evidence of Large-Scale Toroidal Magnetic Fields in the Galactic Center

Abstract

We have observed the linear polarization of 450 µm continuum emission from the Galactic center, using a new polarimetric detector system that is operated on a 2 m telescope at the South Pole. The resulting polarization map extends ∼ 170 pc along the Galactic plane and ∼ 30 pc in Galactic latitude, and thus covers a significant fraction of the central molecular zone. Our map shows that this region is permeated by large-scale toroidal magnetic fields. We consider our results together with radio observations that show evidence for poloidal fields in the Galactic center, and with Faraday rotation observations. We compare all of these observations with the predictions of a magnetodynamic model for the Galactic center that was proposed in order to explain the Galactic Center Radio Lobe as a magnetically driven gas outflow. We conclude that the observations are basically consistent with the model.

Interstellar magnetic fields in the Galactic center region

“…• of the GC conform neither to Novak et al (2003)'s pattern (only 2 have the expected RM sign) nor to the general pattern of the more extended area (only 2 have RM > 0).…”

“…A much larger area about the GC, extending 170 pc in longitude and 30 pc in latitude, was observed in 450 μm polarized emission by Novak et al (2003). Their polarization map clearly shows that the magnetic field threading molecular clouds is, on the whole, approximately parallel to the Galactic plane.…”

“…To reconcile the horizontal field measured in molecular clouds with the poloidal field traced by NRFs, Novak et al (2003) suggested that the large-scale magnetic field near the GC is predominantly poloidal in the diffuse ISM and predominantly toroidal in dense regions along the Galactic plane, where it was sheared out in the azimuthal direction by the differential rotation of the dense gas. Chuss et al (2003) reported further 350 μm polarimetric observations of the central 50 pc of the Galaxy, which led them to refine the conclusions of Novak et al (2003). They found that the measured field direction depends on the molecular gas density in such a way that it is generally parallel to the Galactic plane in high-density regions and generally perpendicular to it in low-density regions.…”

Magnetic fields in our Milky Way Galaxy and nearby galaxies

“…A similar situation occurs for polarization measurements of thermal emission from molecular clouds at mm, submm or infrared wavelengths. Observed magnetic fields in most clouds are parallel to the Galactic plane (Novak et al 2003, Li et al 2006, Bierman et al 2011, no matter how far away or where the clouds are located in the disk, except that the large-scale fields are distorted locally. From such orientation measurements of cloud fields, it is impossible to derive the large-scale magnetic fields in the disk.…”

Proton-temperature-anisotropy-driven magnetic fields in plasmas with cold and relativistically hot electrons

“…The importance of relativistic thermal Weibel instability [13,14] has also been recognized in the context of large-scale magnetic fields in a number of astrophysical sources, such as the gamma-ray bursts and relativistic jets [15,16]. Recently, large-scale toroidal magnetic fields have been observed in the Galactic Centre [17], accompanied with splendid filamentary radio arcs [18].…”