[论文解读] 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 Studies参考文献 113被引用 29

一句话总结

本文利用普朗克2018年全天亚毫米波尘埃极化发射图，分析银河系中尘埃极化的统计特性。结果表明，极化率 $p$ 和极化角弥散度 $S$ 主要受银河系磁场几何结构的支配，满足 $S \propto p^{-1}$ 关系，且极化特性与尘埃温度或尘粒对齐效率无显著依赖，暗示观测到的极化率 $p$ 变化主要由磁场结构决定，而非尘埃物理性质或辐射场变化的影响。

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.

研究动机与目标

• 利用普朗克2018年数据，表征全天空热尘埃发射的极化统计特性。
• 确定极化率 $p$ 和极化角弥散度 $S$ 的变化是否由磁场结构引起，或源于尘粒对齐效率或尘埃性质的变化。
• 通过分析 $p$、$S$ 与尘埃温度 $T_d$ 之间的相关性，检验辐射对齐力矩（RAT）理论的预测。
• 将亚毫米波段尘埃极化与光学星光极化进行比较，约束尘埃消光与辐射特性。
• 推导消光中极化率的上限，并评估其与亚毫米波观测的一致性。

提出的方法

• 利用普朗克2018年数据第三批公开发布产品中353 GHz、80′角分辨率的全天空尘埃极化图。
• 生成全天空的极化率 $p$、极化角 $\psi$ 和极化角弥散函数 $S$ 图像。
• 分析 $p$ 与柱密度 $N_H$ 的联合分布，评估 $p$ 随光学厚度的变化规律。
• 定义乘积 $S \times p$，以分离非几何因素对极化的影响，独立于磁场拓扑结构。
• 将普朗克亚毫米波极化结果与1,505颗高银纬恒星的光学星光极化数据进行比较。
• 计算极化率比值 $R_{P/p}$ 和 $R_{S/V}$，以关联发射与消光特性，并推导 $p_V/E(B-V)$ 的约束条件。

实验结果

研究问题

• RQ1 极化率 $p$ 如何随柱密度 $N_H$ 变化？这反映了何种主导物理机制？
• RQ2 根据辐射对齐力矩（RAT）理论预测，$p$ 或 $S$ 是否系统性依赖于尘埃温度 $T_d$？
• RQ3 极化率 $p$、极化角 $\psi$ 和弥散度 $S$ 的统计特性在多大程度上反映了银河系磁场的结构？
• RQ4 普朗克亚毫米波极化测量与光学星光极化在角度一致性与极化率比值方面有何异同？
• RQ5 数据对消光中最大可能极化率 $p_V/E(B-V)$ 设定了何种约束？

主要发现

• 在353 GHz、80′角分辨率下，观测到的最大极化率为 $p_{\text{max}} = 22.0^{+3.5}_{-1.4}\%$，其不确定性主要来自总强度零点校准。
• 在高银纬区域，$S \propto p^{-1}$ 关系成立，证实极化统计特性主要由磁场拓扑结构决定。
• 在 $N_H = 10^{20}$ cm$^{-2}$ 到 $N_H = 2 \times 10^{22}$ cm$^{-2}$ 范围内，$S \times p$ 仅下降约25%，而 $p$ 下降了3至4倍，表明 $p$ 的下降主要源于磁场几何结构，而非尘粒对齐效率。
• 在稀薄星际介质或盖尔斯堡分子云中，均未发现 $S \times p$ 与尘埃温度 $T_d$ 之间存在系统性关联，与RAT理论预期相矛盾。
• 高 $p$ 与低 $S$ 的视线通常对应更冷的尘埃，表明冷尘与尘粒对齐存在相关性，但并非由辐射场强度驱动的因果关系。
• 在高银纬区域，极化率比值 $R_{P/p} = 5.4$ MJy sr$^{-1}$，$R_{S/V} \approx 4.3$，与部分透明视线的值一致，且 $p_V/E(B-V)$ 的上限为 $\leq 13\%$，显著高于低银纬部分透明区域的9%。