Distant probes of RM structure -- Where is the Faraday Rotation towards the Magellanic Leading Arm?

TL;DR

This study uses diffuse polarised emission to determine whether Faraday Rotation Measures towards the Magellanic Leading Arm are caused by distant circumgalactic medium or local interstellar structures, revealing a local supernova remnant as the likely Faraday screen.

Contribution

Introduces a method to distinguish between local and distant Faraday screens using Galactic diffuse polarisation maps, applied to the Magellanic Leading Arm region.

Findings

The RM excess is likely caused by the Antlia supernova remnant, not the Magellanic Leading Arm.

A steep magnetic field gradient is observed in the foreground SNR.

Overlap between RM excess and the Leading Arm is coincidental.

Abstract

Faraday Rotation Measures (RM) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net Faraday rotation along sight-lines. We introduce a simple test using Galactic diffuse polarised emission that evaluates whether structures evident in RM observations are associated with distant circumgalactic medium (CGM) or foreground interstellar medium (ISM). We focus on the Magellanic Leading Arm region where a clear excess of RM was previously reported. There are two gaseous objects standing out in this direction: the distant Magellanic Leading Arm and the nearby Antlia supernova remnant (SNR). We recognized narrow depolarised filaments in the $2.3\,\rm GHz$ S-band Polarization All Sky Survey (S-PASS) image that overlaps with the reported RM excess. We suggest that there is a steep gradient in Faraday rotation in a foreground screen arising from the Antlia SNR. The estimated strength of the line-of-sight component of the magnetic field is $B_{\parallel}\sim 5\,\rm\mu G$, assuming that the excess of RM is entirely an outcome of the magnetized supernova shell. Our analysis indicates that the overlap between the RM excess and the Magellanic Leading Arm is only a remarkable coincidence. We suggest for future RM grid studies that checking Galactic diffuse polarisation maps is a convenient way to identify local Faraday screens.