Ghost of a Shell: Magnetic Fields of Galactic Supershell GSH 006$-$15$+$7

TL;DR

This study uses radio polarisation data and modelling to measure the magnetic field in a Galactic supershell, revealing weak magnetic fields in a low-density interstellar medium region.

Contribution

It presents a novel method combining radio polarisation observations and background emission modelling to determine magnetic fields in a Galactic supershell.

Findings

Peak line-of-sight magnetic field strength of 2.0 μG.

Probes weak magnetic fields in low-density, partially-ionised ISM.

Provides insights into magnetic field structure in supershells.

Abstract

We identify a counterpart to a Galactic supershell in diffuse radio polarisation, and use this to determine the magnetic fields associated with this object. GSH 006$-$15$+$7 has perturbed the polarised emission at 2.3$\,$GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS), acting as a Faraday screen. We model the Faraday rotation over the shell, and produce a map of Faraday depth over the area across it. Such models require information about the polarised emission behind the screen, which we obtain from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23$\,$GHz to 2.3$\,$GHz, to estimate the synchrotron background behind GSH 006$-$15$+$7. Using the modelled Faraday thickness we determine the magnitude and the plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We find a peak line-of-sight field strength of $|B_\parallel|_\text{peak} = 2.0\substack{+0.01 \\ -0.7}\,\mu$G. Our measurement probes weak magnetic fields in a low-density regime (number densities of $\sim0.4\,$cm$^{-3}$) of the ISM, thus providing crucial information about the magnetic fields in the partially-ionised phase.