Planck 2013 results. XI. All-sky model of thermal dust emission

Abstract: This paper presents an all-sky model of dust emission from the Planck 857, 545 and 353 GHz, and IRAS 100 micron data. Using a modified black-body fit to the data we present all-sky maps of the dust optical depth, temperature, and spectral index over the 353-3000 GHz range. This model is a tight representation of the data at 5 arc min. It shows variations of the order of 30 % compared with the widely-used model of Finkbeiner, Davis, and Schlegel. The Planck data allow us to estimate the dust temperature uniformly over the whole sky, providing an improved estimate of the dust optical depth compared to previous all-sky dust model, especially in high-contrast molecular regions. An increase of the dust opacity at 353 GHz, tau_353/N_H, from the diffuse to the denser interstellar medium (ISM) is reported. It is associated with a decrease in the observed dust temperature, T_obs, that could be due at least in part to the increased dust opacity. We also report an excess of dust emission at HI column densities lower than 10^20 cm^-2 that could be the signature of dust in the warm ionized medium. In the diffuse ISM at high Galactic latitude, we report an anti-correlation between tau_353/N_H and T_obs while the dust specific luminosity, i.e., the total dust emission integrated over frequency (the radiance) per hydrogen atom, stays about constant. The implication is that in the diffuse high-latitude ISM tau_353 is not as reliable a tracer of dust column density as we conclude it is in molecular clouds where the correlation of tau_353 with dust extinction estimated using colour excess measurements on stars is strong. To estimate Galactic E(B-V) in extragalactic fields at high latitude we develop a new method based on the thermal dust radiance, instead of the dust optical depth, calibrated to E(B-V) using reddening measurements of quasars deduced from Sloan Digital Sky Survey data.

• The paper introduces an innovative full-sky model of thermal dust emission using a modified blackbody fit to accurately map dust optical depth, temperature, and spectral index.
• The paper reveals significant spatial variations in dust properties, linking increased opacity and decreased temperature with denser interstellar regions.
• The paper refines Galactic extinction estimates by calibrating thermal dust radiance with quasar reddening, enhancing extragalactic research accuracy.

Overview of "Planck 2013 Results. XI. All-sky Model of Thermal Dust Emission"

The paper presents a comprehensive model of thermal dust emission across the entire sky, based on data from the Planck and IRAS satellites. This model spans frequencies from 353 GHz to 3000 GHz (or wavelengths from 100 to 850 microns), employing a modified blackbody (MBB) fit to produce maps of dust optical depth, temperature, and spectral index. Such parametrization of dust emission improves upon previous models, notably Finkbeiner, Davis, and Schlegel's (FDS) model, by incorporating Planck's higher spectral coverage and angular resolution.

Methodology and Data Analysis

The model uses Planck data at 857, 545, and 353 GHz and integrates it with 100-micron data from IRAS. A two-step fitting procedure is employed to mitigate the impact of noise and cosmic infrared background anisotropies (CIBA) on parameter estimation. Initially, data are smoothed to 30’, fitting for dust spectral index and temperature; subsequently, the optical depth and temperature are refined at 5’ resolution using the spectral index from the initial step.

Key Results and Findings

1. Spatial Variations: The study highlights the variations in dust emission parameters across different Galactic environments. Mean dust temperature across the sky was found to be 19.7 K with standard deviations indicating a range of thermal states influenced by interstellar radiation fields and dust properties.
2. Dust Opacity: An increase in dust opacity at 353 GHz from diffuse to denser interstellar medium (ISM) regions was noted. The rise in opacity is contemporaneous with a decrease in observed dust temperature, suggesting increased dust emissivity in dense media possibly due to dust aggregation or structural changes.
3. Dust Emission and Hydrogen Column Density Relations: In the diffuse ISM, at high latitudes, a constant specific dust luminosity was found, suggesting a fairly uniform interstellar radiation field (ISRF). At lowest column densities, an excess dust emission was detected, potentially due to dust in the warm ionized medium.
4. Implications for Radiative Transfer: The results suggest that dust temperature variations may not directly reflect variations in ISRF strength but could instead indicate variations in dust properties. This calls into question the reliability of using observed dust temperature as a standalone tracer of ISRF.
5. Comparison with Previous Models: The new Planck-based model diverges significantly from the FDS model, particularly in the Galactic plane regions, indicating more accurate and localized retrieval of dust properties with Planck's higher sensitivity and resolution.
6. Extragalactic Applications: One of the practical outputs of this analysis is the estimation of Galactic $E(B-V)$ for extragalactic studies. By calibrating the thermal dust radiance with quasar reddening measurements, an improved map of Galactic extinction was produced for high-latitude regions.

Theoretical and Practical Implications

The paper contributes to understanding dust-grain evolution and its role as both a catalyst and recorder of the physical conditions within the ISM. It challenges previous assumptions by suggesting that dust temperature is modified more by variations in dust properties than by ISRF strength alone. Practically, the model aids in correcting for dust extinction in astrophysical observations, thus improving the accuracy of studies within and beyond our galaxy.

Future Directions

Further investigations into dust properties using more sensitive data or combining with complementary observations (e.g., Herschel) could reveal granular details of ISRF variations and corresponding dust property changes. Additionally, expanding models to cover different cosmic environments could provide insights into the lifecycle and structural evolution of cosmic dust.

This comprehensive model of thermal dust emission is, therefore, a pivotal reference for both interpreting dust-related astrophysical phenomena and refining observational strategies across diverse cosmic environments.