Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust (1405.0871v1)

Abstract: This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium.We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

• The paper quantifies that maximum dust polarization fractions exceed 18% in intermediate column density regions, highlighting Planck's enhanced sensitivity.
• It demonstrates a systematic decline in polarization with increasing dust column density, supporting theories of radiative grain alignment and magnetic field variation.
• The paper maps polarization angle dispersion to reveal ordered magnetic fields interrupted by filamentary structures, offering insights into interstellar magnetic dynamics.

Analysis of Large-Scale Polarized Thermal Emission from Galactic Dust: Insights from Planck Data

The paper presents an analysis of the large-scale polarized sky as observed by the Planck satellite, focusing particularly on the 353 GHz channel, which is most sensitive to dust polarization. This paper provides comprehensive maps of dust polarization fraction and direction, considering noise biases and potential systematic effects.

Key Findings

1. Polarization Fraction and its Variability: The research details that the maximum observed dust polarization fraction is significant, exceeding 18% in specific regions with intermediate dust column density. This level of polarization is higher than that typically reported by ground-based observations, attributable to the high sensitivity and wide sky coverage of the Planck data.
2. Correlation with Dust Column Density: There is a systematic decline in dust polarization fraction with increasing dust column density, interpreted through models of radiative grain alignment and magnetic field orientation variations. Regions with low polarization fractions often correspond to areas with significant fluctuations in the polarization angle.
3. Polarization Angle and Magnetic Field Structure: The paper characterizes the spatial structure of the polarization angle using the angle dispersion function, identifying ordered magnetic fields over extended areas interrupted by filamentary structures. These structures seem to act as interfaces where the magnetic field projection rotates abruptly, often independent of dust column density variations.
4. Comparison with Synchrotron Emission: The paper also compares dust polarization with polarized synchrotron emission, revealing globally similar structures in regions like the Galactic plane and notable areas such as the Fan and North Polar Spur. However, it is highlighted that dust and synchrotron emissions typically sample different regions along the line of sight, confirming that much of the observed Planck data variations are due to the three-dimensional structure of the magnetic field.

Implications

• Theoretical Understanding: This paper contributes significantly to the understanding of interstellar magnetic fields and dust grain alignment mechanisms. The observed maximum polarization fraction sets a lower limit to the intrinsic polarization capabilities of dust grains aligned optimally with the magnetic field, offering constraints for theoretical models of dust and magnetic field interactions.
• Practical Applications: For astrophysical studies, particularly those related to the Cosmic Microwave Background (CMB), understanding the polarization characteristics of Galactic dust is crucial to component separation tasks necessary for accurate CMB interpretations.
• Future Prospects in AI and Data Analysis: The methods used for debiasing and accounting for systematic effects in polarization data are valuable not just in astronomy but can influence AI-driven data analysis tools where bias and noise management are critical. Enhanced AI models could further automate and refine such analyses, particularly in distinguishing astrophysical foregrounds from cosmological signals.

The insights from this paper are essential for both advancing the theoretical models of Galactic dust behavior and magnetic field dynamics, and improving the methodological approaches in analyzing complex astrophysical data sets. As research progresses, similar methodologies might also be adapted for other observational platforms and frequency ranges, broadening the horizon for future discoveries in astrophysics and related fields.