Planck 2018 results. XII. Galactic astrophysics using polarized dust emission (1807.06212v2)

Abstract: We present 353 GHz full-sky maps of the polarization fraction $p$, angle $\psi$, and dispersion of angles $S$ of Galactic dust thermal emission produced from the 2018 release of Planck data. We confirm that the mean and maximum of $p$ decrease with increasing $N_H$. The uncertainty on the maximum polarization fraction, $p_\mathrm{max}=22.0$% at 80 arcmin resolution, is dominated by the uncertainty on the zero level in total intensity. The observed inverse behaviour between $p$ and $S$ is interpreted with models of the polarized sky that include effects from only the topology of the turbulent Galactic magnetic field. Thus, the statistical properties of $p$, $\psi$, and $S$ mostly reflect the structure of the magnetic field. Nevertheless, we search for potential signatures of varying grain alignment and dust properties. First, we analyse the product map $S \times p$, looking for residual trends. While $p$ decreases by a factor of 3--4 between $N_H=10^{20}$ cm$^{-2}$ and $N_H=2\times 10^{22}$ cm$^{-2}$, $S \times p$ decreases by only about 25%, a systematic trend observed in both the diffuse ISM and molecular clouds. Second, we find no systematic trend of $S \times p$ with the dust temperature, even though in the diffuse ISM lines of sight with high $p$ and low $S$ tend to have colder dust. We also compare Planck data with starlight polarization in the visible at high latitudes. The agreement in polarization angles is remarkable. Two polarization emission-to-extinction ratios that characterize dust optical properties depend only weakly on $N_H$ and converge towards the values previously determined for translucent lines of sight. We determine an upper limit for the polarization fraction in extinction of 13%, compatible with the $p_\mathrm{max}$ observed in emission. These results provide strong constraints for models of Galactic dust in diffuse gas.

• The paper confirms the inverse relationship between polarization fraction and its angle dispersion, consistent with the expected p⁻¹ trend.
• The study attributes variations in polarization to the Galactic magnetic field structure rather than changes in dust grain alignment, with maximum fractions reaching about 22% at 353 GHz.
• The research shows that despite a 3- to 4-fold drop in polarization fraction, the product p×S decreases only by about 25%, refining models of the interstellar medium.

Overview of the Planck 2018 Results on Galactic Astrophysics Using Polarized Dust Emission

The paper titled "Planck 2018 results. XII. Galactic astrophysics using polarized dust emission" provides an in-depth analysis of the polarized emission from Galactic dust using the Planck satellite data. This paper presents significant insights into the interstellar medium (ISM) by examining the full-sky maps of dust polarization fraction, angle, and the dispersion function of polarization angles. The research focuses on understanding the structure of the Galactic magnetic field through the statistical properties of polarized thermal emission from dust.

Key Findings and Numerical Results

The paper confirms the inverse relationship between the polarization fraction $p$ and the polarization angle dispersion function $\mathcal{S}$, previously observed in smaller sky regions. This inverse relationship adheres to the model expectations $\mathcal{S} \propto p^{-1}$, where the mean and maximum polarization fractions decrease with increasing column density $N_H$. At a 353 GHz frequency and a resolution of 80 arcminutes, the maximum observed polarization fraction is estimated to be $22.0^{+3.5}_{-1.4}\%$, constrained by uncertainties in total intensity zero level, particularly at high Galactic latitudes.

The analysis also shows that the trends in the polarization fraction with other parameters—such as dust temperature and column density—are predominantly prescribed by the magnetic field structure rather than changes in grain alignment or intrinsic dust properties. While the polarization fraction $p$ decreases by a factor of 3 to 4 in some regions, the product $p \times \mathcal{S}$ decreases by only about 25%, indicating the role of systematic magnetic field structure variations over grain alignment efficiency drops.

Implications and Future Directions

These results provide critical constraints for theoretical models of dust in the ISM. They affirm that magnetic field topology plays a notable role in polarization characteristics, influencing the interpretation of observed polarization data. The lack of systematic trends related to dust temperature suggests that the effect of radiative alignment torques on grain alignment may be less significant than traditionally expected.

This research invites future theoretical work to refine models of the Galactic magnetic field. It also calls for further observational efforts to corroborate these findings across different Galactic environments using next-generation telescopes with higher sensitivity and resolution. Beyond improving ISM models, these insights have implications for accurately interpreting polarized emission from other astrophysical structures, contributing to cosmological studies that utilize dust polarization to disentangle foreground influences in cosmic microwave background analyses.

Concluding Remarks

The paper underscores the power of Planck full-sky maps in advancing our understanding of galactic astrophysics, particularly in revealing the intricate details of the polarization characteristics of interstellar dust. The profound relationship between field structure and polarization lays a foundation for future research endeavors aimed at disentangling the multifaceted dynamics of the Galactic ecosystem.