A Survey of Extragalactic Faraday Rotation at High Galactic Latitude: The Vertical Magnetic Field of the Milky Way towards the Galactic Poles

TL;DR

This study uses Faraday rotation measurements of over 1000 extragalactic sources to investigate the Milky Way's vertical magnetic field at high galactic latitudes, finding no evidence of a coherent vertical magnetic component.

Contribution

It provides the first large-scale survey of the vertical magnetic field at high galactic latitudes, constraining its strength and symmetry, and challenges simple dipole or quadrupole models of the Galactic magnetic field.

Findings

Median RMs near zero at the poles, indicating no coherent vertical field.

No preferred angular scale in RM fluctuations from 0.1 to 25 degrees.

Upper limit of about 1 microGauss on the random magnetic field strength.

Abstract

We present a study of the vertical magnetic field of the Milky Way towards the Galactic poles, determined from observations of Faraday rotation toward more than 1000 polarized extragalactic radio sources at Galactic latitudes |b| > 77 degs, using the Westerbork Radio Synthesis Telescope and the Australia Telescope Compact Array. We find median rotation measures (RMs) of 0.0 +/- 0.5 rad/m^2 and +6.3 +/- 0.7 rad/m^2 toward the north and south Galactic poles, respectively, demonstrating that there is no coherent vertical magnetic field in the Milky Way at the Sun's position. If this is a global property of the Milky Way's magnetism, then the lack of symmetry across the disk rules out pure dipole or quadrupole geometries for the Galactic magnetic field. The angular fluctuations in RM seen in our data show no preferred scale within the range ~ 0.1 to 25 degs. The observed standard deviation in RM of ~ 9 rad/m^2 then implies an upper limit of ~1microGauss on the strength of the random magnetic field in the warm ionized medium at high Galactic latitudes.