Helicity in the Large-Scale Galactic Magnetic Field

TL;DR

This study investigates magnetic helicity in the Milky Way's large-scale magnetic field using all-sky radio polarization data, providing observational evidence supporting the dynamo theory of galactic magnetic field origin.

Contribution

It compares observational data with simulated models to detect magnetic helicity, offering new insights into the field's structure and supporting the dynamo origin hypothesis.

Findings

Evidence for a quadrupolar magnetic field with a vertical component

Large-scale helicity reflected in polarization and Faraday depth

Supports dynamo theory for galactic magnetic fields

Abstract

We search for observational signatures of magnetic helicity in data from all-sky radio polarization surveys of the Milky Way Galaxy. Such a detection would help confirm the dynamo origin of the field and may provide new observational constraints for its shape. We compare our observational results to simulated observations for both a simple helical field, and for a more complex field that comes from a solution to the dynamo equation. Our simulated observations show that the large-scale helicity of a magnetic field is reflected in the large-scale structure of the fractional polarization derived from the observed synchrotron radiation and Faraday depth of the diffuse Galactic synchrotron emission. Comparing the models with the observations provides evidence for the presence of a quadrupolar magnetic field with a vertical component that is pointing away from the observer in both hemispheres of the Milky Way Galaxy. Since there is no reason to believe that the Galactic magnetic field is unusual when compared to other galaxies, this result provides further support for the dynamo origin of large-scale magnetic fields in galaxies.