A Method for a Pseudo-Local Measurement of the Galactic Magnetic Field

TL;DR

This paper introduces a novel approach to estimate the local magnetic field in the Milky Way by utilizing Faraday rotation measurements of HII regions, providing more localized insights into Galactic magnetism.

Contribution

It proposes using HII regions as pseudo-local probes for magnetic field measurements, offering a new constraint on Galactic MHD turbulence and insights into magnetic field modification during star formation.

Findings

HII regions can serve as pseudo-local magnetic field measurements.

New constraints on Galactic MHD turbulence are derived.

Speculations on magnetic field modification during star formation are discussed.

Abstract

Much of the information about the magnetic field in the Milky Way and other galaxies comes from measurements which are path integrals, such as Faraday rotation and the polarization of synchrotron radiation of cosmic ray electrons. The measurement made at the radio telescope results from the contributions of volume elements along a long line of sight. The inferred magnetic field is therefore some sort of average over a long line segment. A magnetic field measurement at a given spatial location is of much more physical significance. In this paper, we point out that HII regions fortuitously offer such a ``point'' measurement, albeit of one component of the magnetic field, and averaged over the sightline through the HII region. However, the line of sight (LOS) through an HII region is much smaller (e.g. 30 - 50 pc) than one through the entire Galactic disk, and thus constitutes a ``pseudo-local'' measurement. We use published HII region Faraday rotation measurements to provide a new constraint on the magnitude of magnetohydrodynamic (MHD) turbulence in the Galaxy, as well as to raise intriguing speculations about the modification of the Galactic field during the star formation process.