Faraday Tomography of the North Polar Spur: Constraints on the distance to the Spur and on the Magnetic Field of the Galaxy

TL;DR

This study uses radio polarization data to analyze the North Polar Spur, constraining its distance and magnetic field properties, and suggests it is a local feature within a few hundred parsecs based on Faraday rotation measurements.

Contribution

It provides new Faraday rotation and depth maps of the NPS, offering constraints on its distance and magnetic field structure, and discusses implications for Galactic magnetic field symmetry.

Findings

The NPS is an emitting-only feature with near-zero Faraday depth.

The NPS is likely a local structure at a distance of a few hundred parsecs.

The Galactic magnetic field may have a coherent vertical component.

Abstract

We present radio continuum and polarization images of the North Polar Spur (NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization angle versus wavelength squared over 2048 frequency channels from 1280 to 1750 MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM map with a published Faraday depth map of the entire Galaxy in this direction, we derive the Faraday depth introduced by the NPS and the Galactic interstellar medium (ISM) in front of and behind the NPS. The Faraday depth contributed by the NPS is close to zero, indicating that the NPS is an emitting only feature. The Faraday depth caused by the ISM in front of the NPS is consistent with zero at b>50 degree, implying that this part of the NPS is local at a distance of approximately several hundred parsecs. The Faraday depth contributed by the ISM behind the NPS gradually increases with Galactic latitude up to b=44 degree, and decreases at higher Galactic latitudes. This implies that either the part of the NPS at b<44 degree is distant or the NPS is local but there is a sign change of the large-scale magnetic field. If the NPS is local, there is then no evidence for a large-scale anti-symmetry pattern in the Faraday depth of the Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes b>50 degree can be explained by including a coherent vertical magnetic field.