This paper presents an overview of the polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse maps of dust polarization fraction and polarization angle at 1° resolution, taking into account noise bias and possible systematic effects. The sensitivity of the Planck HFI polarization measurements allows for the first time a mapping of Galactic dust polarized emission on large scales, including low column density regions. We find that the maximum observed dust polarization fraction is high (p max = 19.8%), in particular in some regions of moderate hydrogen column density (N H < 2 × 1021 cm-2). The polarization fraction displays a large scatter at N H below a few 1021 cm-2. There is a general decrease in the dust polarization fraction with increasing column density above N H ≃ 1 × 1021 cm-2 and in particular a sharp drop above N H ≃ 1.5 × 1022 cm-2. We characterize the spatial structure of the polarization angle using the angle dispersion function. We find that the polarization angle is ordered over extended areas of several square degrees, separated by filamentary structures of high angle dispersion function. These appear as interfaces where the sky projection of the magnetic field changes abruptly without variations in the column density. The polarization fraction is found to be anti-correlated with the dispersion of polarization angles. These results suggest that, at the resolution of 1°, depolarization is due mainly to fluctuations in the magnetic field orientation along the line of sight, rather than to the loss of grain alignment in shielded regions. We also compare the polarization of thermal dust emission with that of synchrotron measured with Planck, low-frequency radio data, and Faraday rotation measurements toward extragalactic sources. These components bear resemblance along the Galactic plane and in some regions such as the Fan and North Polar Spur regions. The poor match observed in other regions shows, however, that dust, cosmic-ray electrons, and thermal electrons generally sample different parts of the line of sight.

Polarized emission observed by Planck HFI at 353 GHz towards a sample of nearby fields is presented, focusing on the statistics of polarization fractions p and angles ψ. The polarization fractions and column densities in these nearby fields are representative of the range of values obtained over the whole sky. We find that: (i) the largest polarization fractions are reached in the most diffuse fields; (ii) the maximum polarization fraction p max decreases with column density N H in the more opaque fields with N H> 1021 cm-2; and (iii) the polarization fraction along a given line of sight is correlated with the local spatial coherence of the polarization angle. These observations are compared to polarized emission maps computed in simulations of anisotropic magnetohydrodynamical turbulence in which we assume a uniform intrinsic polarization fraction of the dust grains. We find that an estimate of this parameter may be recovered from the maximum polarization fraction p max in diffuse regions where the magnetic field is ordered on large scales and perpendicular to the line of sight. This emphasizes the impact of anisotropies of the magnetic field on the emerging polarization signal. The decrease of the maximum polarization fraction with column density in nearby molecular clouds is well reproduced in the simulations, indicating that it is essentially due to the turbulent structure of the magnetic field: an accumulation of variously polarized structures along the line of sight leads to such an anti-correlation. In the simulations, polarization fractions are also found to anti-correlate with the angle dispersion function 𝒮. However, the dispersion of the polarization angle for a given polarization fraction is found to be larger in the simulations than in the observations, suggesting a shortcoming in the physical content of these numerical models. In summary, we find that the turbulent structure of the magnetic field is able to reproduce the main statistical properties of the dust polarization as observed in a variety of nearby clouds, dense cores excluded, and that the large-scale field orientation with respect to the line of sight plays a major role in the quantitative analysis of these statistical properties.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

The Planck survey provides unprecedented full-sky coverage of the submillimetre polarized emission from Galactic dust. In addition to the information on the direction of the Galactic magnetic field, this also brings new constraints on the properties of dust. The dust grains that emit the radiation seen by Planck in the submillimetre also extinguish and polarize starlight in the visible. Comparison of the polarization of the emission and of the interstellar polarization on selected lines of sight probed by stars provides unique new diagnostics of the emission and light scattering properties of dust, and therefore of the important dust model parameters, composition, size, and shape. Using ancillary catalogues of interstellar polarization and extinction of starlight, we obtain the degree of polarization, p V , and the optical depth in the V band to the star, τ V . Toward these stars we measure the submillimetre polarized intensity, P S, and total intensity, I S, in the Planck 353 GHz channel. We compare the column density measure in the visible, E(B − V), with that inferred from the Planck product map of the submillimetre dust optical depth and compare the polarization direction (position angle) in the visible with that in the submillimetre. For those lines of sight through the diffuse interstellar medium with comparable values of the estimated column density and polarization directions close to orthogonal, we correlate properties in the submillimetre and visible to find two ratios, R S /V = (P S/I S) / (p V /τ V ) and R P/p = P S/p V , the latter focusing directly on the polarization properties of the aligned grain population alone.We find R S /V = 4.2, with statistical and systematic uncertainties 0.2 and 0.3, respectively, and R P/p = 5.4 MJy sr-1, with uncertainties 0.2 and 0.3 MJy sr-1, respectively. Our estimate of R S /V is compatible with predictions based on a range of polarizing dust models that have been developed for the diffuse interstellar medium. This estimate provides new empirical validation of many of the common underlying assumptions of the models, but is not yet very discriminating among them. However, our estimate of R P/p is not compatible with predictions, which are too low by a factor of about 2.5. This more discriminating diagnostic, R P/p , indicates that changes to the optical properties in the models of the aligned grain population are required. These new diagnostics, together with the spectral dependence in the submillimetre from Planck,will be important for constraining and understanding the full complexity of the grain models, and for interpreting the Planck thermal dust polarization and refinement of the separation of this contamination of the cosmic microwave background.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Magnetic field plays a crucial role in shaping molecular clouds and regulating star formation, yet the complete information on the magnetic field is not well constrained owing to the limitations in observations. We study the magnetic field in the massive infrared dark cloud G035.39-00.33 from dust continuum polarization observations at 850 μm with SCUBA-2/POL-2 at JCMT for the first time. The magnetic field tends to be perpendicular to the densest part of the main filament (F _M), whereas it has a less defined relative orientation in the rest of the structure, where it tends to be parallel to some diffuse regions. A mean plane-of-the-sky magnetic field strength of ∼50 μG for F _M is obtained using the Davis–Chandrasekhar–Fermi method. Based on ¹³CO (1–0) line observations, we suggest a formation scenario of F _M due to large-scale (∼10 pc) cloud–cloud collision. Using additional NH₃ line data, we estimate that F _M will be gravitationally unstable if it is only supported by thermal pressure and turbulence. The northern part of F _M, however, can be stabilized by a modest additional support from the local magnetic field. The middle and southern parts of F _M are likely unstable even if the magnetic field support is taken into account. We claim that the clumps in F _M may be supported by turbulence and magnetic fields against gravitational collapse. Finally, we identified for the first time a massive (∼200 M _⊙), collapsing starless clump candidate, “c8,” in G035.39-00.33. The magnetic field surrounding “c8” is likely pinched, hinting at an accretion flow along the filament.