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Planck 2018 results: XII. Galactic astrophysics using polarized dust emission

Aghanim, N; Akrami, Y; Alves, MIR; Ashdown, M; Aumont, J; Baccigalupi, C; Ballardini, M; ... et alia, .; + view all (2020) Planck 2018 results: XII. Galactic astrophysics using polarized dust emission. Astronomy & Astrophysics , 641 , Article A12. 10.1051/0004-6361/201833885. Green open access

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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.

Type: Article
Title: Planck 2018 results: XII. Galactic astrophysics using polarized dust emission
Open access status: An open access version is available from UCL Discovery
DOI: 10.1051/0004-6361/201833885
Publisher version: https://doi.org/10.1051/0004-6361/201833885
Language: English
Additional information: © Planck Collaboration 2020. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0).
Keywords: olarization, magnetic fields, turbulence, dust, extinction, local insterstellar matter, submillimeter: ISM
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/10110660
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