[Paper Review] Observations of Polarized Dust Emission at Far-infrared through Millimeter Wavelengths

John E. Vaillancourt|ArXiv.org|Apr 13, 2009

Atmospheric aerosols and clouds2 references3 citations

TL;DR

This paper reviews polarized dust emission at far-infrared to millimeter wavelengths, demonstrating that thermal emission from magnetically aligned dust grains traces interstellar magnetic fields across scales from entire galaxies to protostellar disks. Key findings include the detection of orthogonal polarization signatures between emission and extinction, and the need for multi-wavelength, high-resolution observations to constrain grain alignment and cloud structure models.

ABSTRACT

Interstellar polarization at far-infrared through millimeter wavelengths (0.1 - 1 mm) is primarily due to thermal emission from dust grains aligned with magnetic fields. This mechanism has led to studies of magnetic fields in a variety of celestial sources, as well as the physical characteristics of the dust grains and their interaction with the field. Observations have covered a diverse array of sources, from entire galaxies to molecular clouds and proto-stellar disks. Maps have been generated on a wide range of angular scales, from surveys covering large fractions of the sky, down to those with arcsecond spatial resolution. Additionally, the increasing availability of observations at multiple wavelengths in this band allows empirical tests of models of grain alignment and cloud structure. I review some of the recent work in this field, emphasizing comparisons of observations on multiple spatial scales and at multiple wavelengths.

Motivation & Objective

• To synthesize recent observations of polarized dust emission across far-infrared to millimeter wavelengths to understand interstellar magnetic field structures.
• To investigate how dust grain alignment mechanisms influence polarization signatures in different interstellar environments.
• To identify gaps in current data, particularly in wavelength coverage and spatial resolution, to improve modeling of grain alignment and cloud structure.
• To compare polarization data across multiple angular scales, from arcsecond to degree scales, to trace field evolution from galactic to local cloud scales.
• To advocate for future high-resolution, multi-wavelength observations to test polarization spectrum models and constrain dust properties and magnetic field morphology.

Proposed method

• Compilation and analysis of existing polarimetric observations from single-dish telescopes, interferometers, and space-based surveys across 0.1–1 mm wavelengths.
• Use of interferometric techniques (e.g., Submillimeter Array, ALMA) to achieve arcsecond-scale resolution in star-forming regions.
• Comparison of polarization data at different wavelengths to infer the polarization spectrum and test models of grain alignment and emissivity.
• Integration of data from all-sky surveys (e.g., WMAP, Planck) with high-resolution maps to bridge large-scale and small-scale magnetic field structures.
• Application of models incorporating modern grain alignment theory (e.g., Lazarian 2007) to interpret observed polarization minima near 350 μm.
• Use of multi-wavelength imaging (e.g., Spitzer 3.6 μm vs. Hertz 350 μm) to disentangle warm dust emission from cold dust polarization.

Experimental results

Research questions

• RQ1 How does the polarization spectrum of dust emission vary across different interstellar environments, and what does it reveal about grain alignment and emissivity?
• RQ2 To what extent do magnetic field structures inferred from submillimeter polarimetry correlate with those traced by optical starlight polarization in diffuse media?
• RQ3 How do magnetic field morphologies differ across scales—from galactic plane to protostellar disks—and what physical processes drive these variations?
• RQ4 Why is the polarization spectrum observed to dip near 350 μm, and can this be explained by correlations between dust temperature, emissivity, and alignment efficiency?
• RQ5 What role do high-resolution instruments (e.g., ALMA, SHARP) play in resolving individual stars and distinguishing warm from cold dust emission in dense regions?

Key findings

• Polarized thermal emission at far-infrared to millimeter wavelengths arises from dust grains aligned with magnetic fields, providing a direct probe of magnetic field structure.
• Observations show orthogonal polarization signatures between emission (perpendicular to B) and extinction (parallel to B), confirming the physical mechanism of grain alignment.
• The polarization spectrum exhibits a minimum near 350 μm, suggesting correlations between polarization, dust temperature, and emissivity in molecular clouds.
• High-resolution polarimetry (e.g., 10–20 arcseconds) reveals that cold dust emission is unresolved in dense regions, while warmer dust near embedded stars shows higher contrast in shorter-wavelength FIR data.
• All-sky surveys like WMAP at 94 GHz (3.2 mm) trace large-scale Galactic magnetic fields aligned with the Galactic plane, consistent with optical polarimetry.
• Future instruments with improved spatial resolution (e.g., ALMA, subarcsecond FIR polarimeters) and broader wavelength coverage are essential to test polarization spectrum models and resolve field structure across scales.