Constraints from Faraday rotation on the magnetic field structure in the Galactic halo

TL;DR

This study uses Faraday rotation measures and analytical magnetic field models to constrain the structure of the Galactic halo's magnetic field, suggesting it is likely bisymmetric and X-shaped or axisymmetric and planar.

Contribution

It introduces a method to fit magnetic field models to Faraday rotation data using MCMC, providing new constraints on the halo's magnetic field geometry.

Findings

Galactic halo magnetic field is more likely bisymmetric than axisymmetric.

If bisymmetric, the field appears X-shaped in polarization maps.

If axisymmetric, the field appears nearly parallel to the Galactic plane.

Abstract

We examine the constraints imposed by Faraday rotation measures of extragalactic point sources on the structure of the magnetic field in the halo of our Galaxy. Guided by radio polarization observations of external spiral galaxies, we look in particular into the possibility that field lines in the Galactic halo have an X shape. We employ the analytical models of spiraling, possibly X-shape magnetic fields derived in a previous paper to generate synthetic all-sky maps of the Galactic Faraday depth, which we fit to an observational reference map with the help of Markov Chain Monte Carlo simulations. We find that the magnetic field in the Galactic halo is slightly more likely to be bisymmetric (azimuthal wavenumber, $m = 1$) than axisymmetric ($m = 0$). If it is indeed bisymmetric, it must appear as X-shaped in radio polarization maps of our Galaxy seen edge-on from outside, but if it is actually axisymmetric, it must instead appear as nearly parallel to the Galactic plane.