New Constraints on the Galactic Halo Magnetic Field using Rotation Measures of Extragalactic Sources Towards the Outer Galaxy

TL;DR

This study uses Faraday rotation measures of extragalactic sources to map the Milky Way's halo magnetic field, revealing a symmetric, non-reversing large-scale field with spiral-like structure in the outer Galaxy.

Contribution

It provides new constraints on the Galactic halo magnetic field structure, suggesting a non-reversing, spiral-like configuration based on extensive RM observations.

Findings

Halo magnetic field does not reverse across the Galactic mid-plane.

Magnetic field strength is approximately 2 microGauss at 0.8-2 kpc above the plane.

RM variation indicates a spiral-like halo magnetic field similar to M51.

Abstract

We present a study of the Milky Way disk and halo magnetic field, determined from observations of Faraday rotation measure (RM) towards 641 polarized extragalactic radio sources in the Galactic longitude range 100-117 degs, within 30 degs of the Galactic plane. For |b| < 15 degs, we observe a symmetric RM distribution about the Galactic plane. This is consistent with a disk field in the Perseus arm of even parity across the Galactic mid-plane. In the range 15<|b|<30 degs, we find median rotation measures of -15+/-4 rad/m^2 and -62+/-5 rad/m^2 in the northern and southern Galactic hemispheres, respectively. If the RM distribution is a signature of the large-scale field parallel to the Galactic plane, this suggests that the halo magnetic field toward the outer Galaxy does not reverse direction across the mid-plane. The variation of RM as a function of Galactic latitude in this longitude range is such that RMs become more negative at larger |b|. This is consistent with an azimuthal magnetic field of strength 2 microGauss (7 microGauss) at a height 0.8-2 kpc above (below) the Galactic plane between the local and the Perseus spiral arm. We propose that the Milky Way could possess spiral-like halo magnetic fields similar to those observed in M51.