Mapping the Magnetic Interstellar Medium in Three Dimensions Over the Full Sky with Neutral Hydrogen (1909.11673v1)

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.

• The paper demonstrates that using H I 4-PI survey data and the Rolling Hough Transform yields precise 3D maps of the Galactic magnetic field.
• It reveals strong correlations, with Pearson coefficients over 0.75, between H I-derived polarization parameters and Planck dust emission.
• The findings establish a novel framework for studying ISM dynamics and improving CMB foreground models via detailed magnetic mapping.

Overview of 3D Mapping of the Magnetic Interstellar Medium Using Neutral Hydrogen

In their research, Clark and Hensley address one of the complexities in astrophysics: mapping the three-dimensional magnetic field structure within the interstellar medium (ISM) of our galaxy. Specifically, their work leverages the emission characteristics of 21-cm neutral hydrogen (H I) to create comprehensive maps detailing the magnetic field's geometry across the sky.

Core Methodology and Data Utilization

The authors utilize the H I 4-PI (H4PI) full-sky survey, with its angular resolution of 16.2 arcminutes, to analyze H I gas's spectral and spatial properties. This survey is remarkably detailed and provides a rich dataset to infer the magnetic field orientations from H I emissions. The study derives insights from the alignment of H I structures with magnetic fields using the Rolling Hough Transform (RHT) method. This technique converts spatial structures observed in H I into quantifiable orientation data related to the local magnetic field, captured across multiple velocities.

Clark and Hensley create three-dimensional Stokes parameter maps ($I$, $Q$, $U$) in position-position-velocity (PPV) space, allowing for a more nuanced understanding of magnetic coherence based on H I observations. This analysis is juxtaposed with Planck satellite dust polarization data at 353 GHz, presenting a compelling argument for the correlation between the two different observational domains.

Numerical Results and Model Evaluation

The study presents robust correlations between H I-derived polarization parameters and observed dust emission properties, with notable statistical measures such as a Pearson correlation coefficient exceeding 0.75 for certain integrated quantities between the H I and Planck data. A key aspect is the reproduced asymmetry between $E$- and $B$-mode polarizations, mirroring the Planck findings. This corroboration with observational data suggests the presence of a well-aligned spatial magnetometry provided by the structured H I medium.

Furthermore, Clark and Hensley introduce and validate a magnetically coherent cloud model that parses these parameters into distinct structures within the H I density and velocity space. It confirms that examining velocity-orientation distributions can qualitatively and quantitatively improve predictions of dust emission polarization fractions.

Implications and Future Directions

The implications of this work for astrophysics and cosmology are extensive. The three-dimensional mapping strategy outlined can significantly enhance the understanding of Galactic magnetic field dynamics and interactions, influencing how cosmic rays propagate, magnetic turbulence is understood, and star formation regions evolve. Practically, these H I-based methodologies could offer an empirical foundation to refine Galactic models employed in cosmological measurements, specifically concerning Cosmic Microwave Background (CMB) polarization studies, where dust foreground poses a substantial challenge.

Looking toward future work, there's noteworthy potential in extending these maps to differentiate between ISM phases and conduct longitudinal studies across different Galactic velocities and orientations to categorize magnetic field interactions. This research lays essential groundwork for integrating multiple observational data sources—such as radio continuum and optical polarimetry—into a unified model capable of yielding precise, localized insights into the ISM’s complex structure.

In summary, Clark and Hensley's paper advances a sophisticated and empirically backed approach for mapping our galaxy's magnetic interstellar medium using neutral hydrogen. The outcomes not only enhance theoretical astrophysical models but also provide key data assets for observational cosmology, especially in the ongoing development of CMB experiments.