Mapping the Magnetic Interstellar Medium in Three Dimensions Over the Full Sky with Neutral Hydrogen

TL;DR

This paper introduces a new method to map the three-dimensional structure of the Galactic magnetic field using HI emission data, revealing strong correlations with dust polarization and providing insights into the magnetic field's orientation.

Contribution

The study develops a novel 3D mapping technique of the interstellar magnetic field by analyzing HI filament orientations and comparing them with dust polarization data, without relying on free parameters.

Findings

High correlation ($r > 0.75$) between HI-derived Stokes maps and Planck dust polarization maps.

The $E/B$ ratio from HI maps matches dust emission at large scales, indicating coupling of density structures and magnetic fields.

The 3D maps effectively constrain the structure and orientation of the Galactic magnetic field.

Abstract

Recent analyses of 21-cm neutral hydrogen (HI) emission have demonstrated that HI gas is organized into linear filamentary structures that are preferentially aligned with the local magnetic field, and that the coherence of these structures in velocity space traces line-of-sight magnetic field tangling. On this basis, we introduce a paradigm for modeling the properties of magnetized, dusty regions of the interstellar medium, using the orientation of HI structure at different velocities to map "magnetically coherent" regions of space. We construct three-dimensional (position-position-velocity) Stokes parameter maps using HI4PI full-sky spectroscopic HI data. We compare these maps, integrated over the velocity dimension, to Planck maps of the polarized dust emission at 353 GHz. Without any free parameters governing the relation between HI intensity and dust emission, we find that our $Q$ and $U$ maps are highly correlated ($r > 0.75$) with the 353 GHz $Q$ and $U$ maps of polarized dust emission observed by Planck and reproduce many of its large-scale features. The $E/B$ ratio of the dust emission maps agrees well with the HI-derived maps at large angular scales ($\ell \lesssim 120$), supporting the interpretation that this asymmetry arises from the coupling of linear density structures to the Galactic magnetic field. We demonstrate that our 3D Stokes parameter maps constrain the 3D structure of the Galactic interstellar medium and the orientation of the interstellar magnetic field.