A new window into the sub-parsec scale magnetic field in the Milky Way? Unveiling small-scale magneto-ionic structures with Faraday complexity

TL;DR

This study uses broadband radio polarimetric data of extragalactic sources to reveal small-scale magnetic structures in the Milky Way, demonstrating the effectiveness of Faraday complexity analysis in probing galactic magneto-ionic turbulence.

Contribution

It introduces the FD spread parameter to quantify Faraday depth fluctuations and shows that small-scale Galactic magnetic structures dominate the observed Faraday complexity.

Findings

FD spread peaks near the Galactic mid-plane

Faraday complexity shows longitude modulations

No correlation between source size and FD spread

Abstract

Radio broadband spectro-polarimetric observations are sensitive to the spatial fluctuations of the Faraday depth (FD) within the telescope beam. Such FD fluctuations are referred to as "Faraday complexity", and can unveil small-scale magneto-ionic structures in both the synchrotron-emitting and the foreground volumes. We explore the astrophysical origin of the Faraday complexity exhibited by 191 polarised extragalactic radio sources (EGSs) within 5 deg from the Galactic plane in the longitude range of 20-52 deg, using broadband data from the Karl G. Jansky Very Large Array presented by a previous work. A new parameter called the FD spread is devised to quantify the spatial FD fluctuations. We find that the FD spread of the EGSs (i) demonstrates an enhancement near the Galactic mid-plane, most notable within Galactic latitude of +-3 deg, (ii) exhibits hints of modulations across Galactic longitude, (iii) does not vary with the source size across the entire range of 2.5"-300", and (iv) has an amplitude higher than expected from magneto-ionic structures of extragalactic origin. All these suggest that the primary cause of the Faraday complexity exhibited by our target EGSs is <2.5"-scale magneto-ionic structures in the Milky Way. We argue that the anisotropic turbulent magnetic field generated by galactic-scale shocks and shears, or the stellar feedback-driven isotropic turbulent magnetic field, are the most likely candidates. Our work highlights the use of broadband radio polarimetric observations of EGSs as a powerful probe of multi-scale magnetic structures in the Milky Way.