In:
Astronomy & Astrophysics, EDP Sciences, Vol. 641 ( 2020-09), p. A12-
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 ฯ , and dispersion function of polarization angles ๐ฎ . 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 โ1.4 +3.5 % at 353 GHz and 80โฒ 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 ๐ฎ found earlier is seen to be present at high latitudes. This follows the ๐ฎ โโโ p โ1 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 , ฯ , and ๐ฎ 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 ๐ฎ โ
รโ
p , looking for residual trends. While the polarization fraction p decreases by a factor of 3โ4 between N H โ=โ10 20 โcm โ2 and N H โ=โ2โ
รโ
10 22 โcm โ2 , out of the Galactic plane, this product ๐ฎ โ
รโ
p only decreases by about 25%. Because ๐ฎ 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 ๐ฎ โ
รโ
p with the dust temperature T d , 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 ๐ฎ tend, on the contrary, to have colder dust than lines of sight with low p and high ๐ฎ . 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 โ
โโ
V ), of 13% at high Galactic latitude, compatible with the polarization fraction p โโโ20% observed at 353 GHz. Taken together, these results provide strong constraints for models of Galactic dust in diffuse gas.
Type of Medium:
Online Resource
ISSN:
0004-6361
,
1432-0746
DOI:
10.1051/0004-6361/201833885
Language:
English
Publisher:
EDP Sciences
Publication Date:
2020
detail.hit.zdb_id:
1458466-9
SSG:
16,12
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