Strong excess Faraday rotation on the Inside of the Sagittarius spiral arm

TL;DR

This study reveals strong Faraday rotation signals within the Sagittarius spiral arm, indicating a significant magnetic field influence in the diffuse ionized medium, which impacts models of Galactic magnetism and extragalactic radio burst observations.

Contribution

First detailed measurement of high Faraday rotation within the Sagittarius arm, highlighting the role of diffuse plasma as a Faraday screen in the inner Galaxy.

Findings

High RM values concentrated near the Sagittarius arm tangent.

RM decreases rapidly outside the arm region.

Diffuse Warm Ionized Medium is the primary Faraday screen.

Abstract

We present first results for Faraday rotation of compact polarized sources (1 to 2 GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner Galaxy. In the Galactic longitude range 39 degr < l < 52 degr, we find rotation measures in the range -310 rad/m2 < RM < +4219 rad/m2, with the highest values concentrated within a degree of l = 48 degrees at the Sagittarius arm tangent. Most of the high RMs arise in diffuse plasma, along lines of sight that do not intersect HII regions. For l > 49 degr, RM drops off rapidly, while at l < 47 degr, the mean RM is higher with a larger standard deviation than at l > 49 degr. We attribute the RM structure to the compressed diffuse Warm Ionized Medium in the spiral arm, upstream of the major star formation regions. The Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of Fast Radio Bursts from their host galaxies. We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys.