[Paper Review] Planck 2018 results - XII. Galactic astrophysics using polarized dust emission

M. Bersanelli, L. P. L. Colombo+160 more|arXiv (Cornell University)|Jan 1, 2018

Astrophysics and Star Formation StudiesPhysics and Astronomy113 references29 citations

TL;DR

This paper uses Planck 2018 full-sky submillimetre maps of polarized dust emission to analyze the statistical properties of dust polarization across the Milky Way. It demonstrates that the polarization fraction $p$ and angle dispersion $S$ are primarily governed by the geometry of the Galactic magnetic field, with $S \propto p^{-1}$, and finds no significant dependence of polarization on dust temperature or grain alignment efficiency—implying that observed trends in $p$ are dominated by magnetic field structure rather than dust physics or radiation field variations.

ABSTRACT

Observations of the submillimetre emission from Galactic dust, in both total intensity I and polarization, have received tremendous interest thanks to
 the Planck full-sky maps. In this paper we make use of such full-sky maps of dust polarized emission produced from the third public release of Planck
 data. As the basis for expanding on astrophysical studies of the polarized thermal emission from Galactic dust, we present full-sky maps of the dust
 polarization fraction p, polarization angle ψ, and dispersion function of polarization angles S. The joint distribution (one-point statistics) of p and N_{H} confirms that the mean and maximum polarization fractions decrease with increasing N_{H}. The uncertainty on the maximum observed polarization
 fraction, p_{max} = 22.0^{+3.5}_{−1.4}% at 353 GHz and 800
 resolution, is dominated by the uncertainty on the Galactic emission zero level in total intensity, in particular towards diffuse lines of sight at high Galactic latitudes. Furthermore, the inverse behaviour between p and S found earlier is seen to be present at high latitudes. This follows the S ∝ p^{−1} relationship expected from models of the polarized sky (including numerical simulations of
 magnetohydrodynamical turbulence) that include effects from only the topology of the turbulent magnetic field, but otherwise have uniform alignment
 and dust properties. Thus, the statistical properties of p, ψ, and S for the most part reflect the structure of the Galactic magnetic field. Nevertheless,
 we search for potential signatures of varying grain alignment and dust properties. First, we analyse the product map S × p, looking for residual trends. While the polarization fraction p decreases by a factor of 3−4 between NH = 10^{20} cm^{−2} and N_{H} = 2 × 10^{22} cm^{−2}, out of the Galactic plane,
 this product S × p only decreases by about 25%. Because S is independent of the grain alignment efficiency, this demonstrates that the systematic decrease in p with N_{H} is determined mostly by the magnetic-field structure and not by a drop in grain alignment. This systematic trend is observed
 both in the diffuse interstellar medium (ISM) and in molecular clouds of the Gould Belt. Second, we look for a dependence of polarization properties
 on the dust temperature, as we would expect from the radiative alignment torque (RAT) theory. We find no systematic trend of S × p with the dust
 temperature Td, whether in the diffuse ISM or in the molecular clouds of the Gould Belt. In the diffuse ISM, lines of sight with high polarization
 fraction p and low polarization angle dispersion S tend, on the contrary, to have colder dust than lines of sight with low p and high S. We also
 compare the Planck thermal dust polarization with starlight polarization data in the visible at high Galactic latitudes. The agreement in polarization
 angles is remarkable, and is consistent with what we expect from the noise and the observed dispersion of polarization angles in the visible on the
 scale of the Planck beam. The two polarization emission-to-extinction ratios, R_{P/p} and R_{S/V}, which primarily characterize dust optical properties,
 have only a weak dependence on the column density, and converge towards the values previously determined for translucent lines of sight. We also
 determine an upper limit for the polarization fraction in extinction, p_{V}/E(B − V), of 13% at high Galactic latitude, compatible with the polarization
 fraction p ≈ 20% observed at 353 GHz. Taken together, these results provide strong constraints for models of Galactic dust in diffuse gas.

Motivation & Objective

• To characterize the full-sky statistical properties of polarized thermal dust emission using Planck 2018 data.
• To determine whether variations in dust polarization fraction $p$ and angle dispersion $S$ are driven by magnetic field structure or by changes in grain alignment or dust properties.
• To test predictions of radiative alignment torque (RAT) theory by examining correlations between $p$, $S$, and dust temperature $T_d$.
• To compare submillimetre dust polarization with optical starlight polarization, constraining dust extinction and emission properties.
• To derive upper limits for the polarization fraction in extinction and assess consistency with submillimetre observations.

Proposed method

• Utilized full-sky maps of dust polarization from the third public release of Planck 2018 data at 353 GHz and 80′ resolution.
• Produced full-sky maps of polarization fraction $p$, polarization angle $\psi$, and polarization angle dispersion function $S$.
• Analyzed the joint distribution of $p$ and column density $N_H$ to assess how $p$ varies with optical depth.
• Defined the product $S \times p$ to isolate non-geometric contributions to polarization, independent of magnetic field topology.
• Compared Planck submillimetre polarization with optical starlight polarization data from 1,505 high Galactic latitude stars.
• Calculated polarization ratios $R_{P/p}$ and $R_{S/V}$ to link emission and extinction properties, and derived constraints on $p_V/E(B-V)$.

Experimental results

Research questions

• RQ1 How does the polarization fraction $p$ vary with column density $N_H$, and what does this imply about the dominant physical mechanisms?
• RQ2 Is there a systematic dependence of $p$ or $S$ on dust temperature $T_d$, as predicted by radiative alignment torque (RAT) theory?
• RQ3 To what extent do the statistical properties of $p$, $\psi$, and $S$ reflect the structure of the Galactic magnetic field?
• RQ4 How do Planck submillimetre polarization measurements compare with optical starlight polarization in terms of angle agreement and polarization ratios?
• RQ5 What constraints does the data place on the maximum possible polarization fraction in extinction, $p_V/E(B-V)$?

Key findings

• The maximum observed polarization fraction is $p_{\text{max}} = 22.0^{+3.5}_{-1.4}\%$ at 353 GHz and 80′ resolution, with uncertainty dominated by the total intensity zero-level calibration.
• The inverse relationship $S \propto p^{-1}$ holds at high Galactic latitudes, confirming that polarization statistics are primarily shaped by magnetic field topology.
• The product $S \times p$ decreases by only about 25% between $N_H = 10^{20}$ cm$^{-2}$ and $N_H = 2 \times 10^{22}$ cm$^{-2}$, while $p$ decreases by a factor of 3–4, indicating that the drop in $p$ is dominated by magnetic field geometry, not grain alignment efficiency.
• No systematic trend is found between $S \times p$ and dust temperature $T_d$ in either the diffuse ISM or Gould Belt molecular clouds, contradicting expectations from RAT theory.
• Lines of sight with high $p$ and low $S$ tend to have colder dust than those with low $p$ and high $S$, indicating a correlation between cold dust and aligned grains, but not a causal dependence on radiation field intensity.
• The polarization ratios $R_{P/p} = 5.4$ MJy sr$^{-1}$ and $R_{S/V} \approx 4.3$ at high Galactic latitudes are consistent with values for translucent lines of sight, and the upper limit for $p_V/E(B-V)$ is $\leq 13\%$, significantly higher than the 9% value for low-latitude translucent regions.