[Paper Review] Astro2020 Science White Paper: Magnetic Fields and Polarization in the Diffuse Interstellar Medium

Susan E. Clark, Carl Heiles+1 more|arXiv (Cornell University)|Mar 18, 2019

Astrophysics and Star Formation StudiesPhysics and Astronomy4 references3 citations

TL;DR

This white paper advocates for a statistically robust survey of magnetic field strengths in the diffuse interstellar medium using 21-cm line emission and absorption to measure Zeeman splitting, while reporting a serendipitous discovery of linear polarization in the 21-cm line—potentially linked to anisotropic radiation or magnetic sublevel population—requiring further theoretical and observational study. The work emphasizes high dynamic range observations and single-dish polarimetry to decode magnetic field structure from H i morphology.

ABSTRACT

Magnetism is one of the most important forces on the interstellar medium (ISM), anisotropically regulating the structure and star formation that drive galactic evolution. Recent high dynamic range observations of diffuse gas and molecular clouds have revealed new links between interstellar structures and the ambient magnetic field. ISM morphology encodes rich physical information, but deciphering it requires high-resolution measurements of the magnetic field: linear polarization of starlight and dust emission, and Zeeman splitting. These measure different components of the magnetic field, and crucially, Zeeman splitting is the only way to directly measure the field strength in the ISM. We advocate a statistically meaningful survey of magnetic field strengths using the 21-cm line in absorption, as well as an observational test of the link between structure formation and field strength using the 21-cm line in emission. Finally, we report on the serendipitous discovery of linear polarization of the 21-cm line, which demands both theoretical and observational follow-up.

Motivation & Objective

• To enable statistically meaningful measurements of magnetic field strengths in the diffuse interstellar medium (ISM) using 21-cm line Zeeman splitting in emission and absorption.
• To test the link between ISM structure formation and magnetic field strength through high-resolution 21-cm line emission surveys.
• To investigate the astrophysical origin of a serendipitously discovered linear polarization in the 21-cm line, which challenges the assumption of unpolarized collisionally populated two-level systems.
• To develop and apply advanced algorithms for quantifying spatial structure in H i emission to decode magnetic field orientation and strength.
• To promote all-sky, high-resolution polarimetric surveys of dust, starlight, and H i to map magnetic field morphology across the Galaxy.

Proposed method

• Utilize high dynamic range 21-cm line observations from single-dish telescopes such as Arecibo, Parkes, and Effelsberg to measure Zeeman splitting in H i emission and absorption, which directly probes magnetic field strength and direction.
• Apply sensitive, high-sensitivity polarization calibration techniques to Stokes I, Q, and U parameters of the 21-cm line to detect circular and linear polarization, especially in long-integration data from the Heiles & Troland Millennium survey.
• Leverage the Arecibo ALPACA phased-array feed to simultaneously observe 39 off-source and one on-source spectrum per pointing, enabling efficient and accurate measurement of extended H i emission and polarization.
• Combine data from H i fibers, polarized dust emission (e.g., Planck), and optical starlight polarization to cross-validate magnetic field orientation and infer 3D field structure.
• Use advanced spatial structure quantification methods (e.g., from Soler et al. 2013) to correlate filamentary H i morphology with magnetic field alignment and strength.
• Investigate theoretical mechanisms for 21-cm line linear polarization, including anisotropic Ly-α radiation effects on magnetic sublevels, as proposed by Yan & Lazarian (2007).

Experimental results

Research questions

• RQ1 Can a statistically significant survey of 21-cm line Zeeman splitting in H i emission and absorption provide robust, large-scale measurements of magnetic field strength in the diffuse ISM?
• RQ2 To what extent do magnetically aligned H i fibers and filamentary structures trace the plane-of-sky magnetic field orientation, and how can their morphology be quantitatively linked to field strength?
• RQ3 What is the physical origin of the serendipitously detected linear polarization (0.14–0.35%) in the 21-cm line, and does it arise from anisotropic radiation coupling to magnetic sublevels?
• RQ4 How do shock-compressed H i structures in superbubbles, such as in the Orion/Eridanus region, reflect magnetic field reversals or enhanced field strengths?
• RQ5 Can all-sky, high-resolution polarimetric surveys of dust, starlight, and H i emission jointly constrain the 3D magnetic field structure of the Galaxy?

Key findings

• A serendipitous detection of linear polarization in the 21-cm line was found in 13 out of 18 sources from the Heiles & Troland Millennium survey, with polarization levels ranging from 0.14% to 0.35%, challenging the assumption of unpolarized collisionally dominated two-level systems.
• The linear polarization in the 21-cm line is not exclusively associated with absorption (CNM) or emission (WIM and CNM) profiles, indicating a complex origin involving spatial or spectral anisotropy.
• Zeeman splitting measurements in H i emission, such as those from Heiles (1989), show field strengths approximately twice those of random positions, indicating enhanced fields in shock-compressed structures like superbubble shells.
• High dynamic range H i observations reveal that high aspect ratio H i fibers (up to 100:1) are preferentially aligned with the plane-of-sky magnetic field as traced by dust and starlight polarization.
• The 21-cm line’s linear polarization is not purely instrumental, as confirmed by careful calibration and statistical uncertainty analysis, suggesting an astrophysical origin requiring theoretical modeling.
• Single-dish telescopes such as Arecibo, Parkes, and Effelsberg are uniquely suited for measuring spatially extended, polarized H i emission due to their ability to recover total flux, unlike interferometric arrays.