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

Author(s): Collaboration, P; Aghanim, N; Akrami, Y; Alves, MIR; Ashdown, M; Aumont, J; Baccigalupi, C; Ballardini, M; Banday, AJ; Barreiro, RB; Bartolo, N; Basak, S; Benabed, K; Bernard, J-P; Bersanelli, M; Bielewicz, P; Bock, JJ; Bond, JR; Borrill, J; Bouchet, FR; Boulanger, F; Bracco, A; Bucher, M; Burigana, C; Calabrese, E; Cardoso, J-F; Carron, J; Chary, R-R; Chiang, HC; Colombo, LPL; Combet, C; Crill, BP; Cuttaia, F; Bernardis, PD; Zotti, GD; Delabrouille, J; Delouis, J-M; Valentino, ED; Dickinson, C; Diego, JM; Dore, O; Douspis, M; Ducout, A; Dupac, X; Efstathiou, G; Elsner, F; Enslin, TA; Eriksen, HK; Fantaye, Y; Fernandez-Cobos, R; Ferriere, K; Forastieri, F; Frailis, M; Fraisse, AA; Franceschi, E; Frolov, A; Galeotta, S; Galli, S; Ganga, K; Genova-Santos, RT; Gerbino, M; Ghosh, T; Gonzalez-Nuevo, J; Gorski, KM; Gratton, S; Green, G; Gruppuso, A; Gudmundsson, JE; Guillet, V; Handley, W; Hansen, FK; Helou, G; Herranz, D; Hivon, E; Huang, Z; Jaffe, AH; Jones, WC; Keihanen, E; Keskitalo, R; Kiiveri, K; Kim, J; Krachmalnicoff, N; Kunz, M; Kurki-Suonio, H; Lagache, G | Abstract: We present 353 GHz full-sky maps of the polarization fraction $p$, angle $\psi$, and dispersion of angles $S$ of Galactic dust thermal emission produced from the 2018 release of Planck data. We confirm that the mean and maximum of $p$ decrease with increasing $N_H$. The uncertainty on the maximum polarization fraction, $p_\mathrm{max}=22.0$% at 80 arcmin resolution, is dominated by the uncertainty on the zero level in total intensity. The observed inverse behaviour between $p$ and $S$ is interpreted with models of the polarized sky that include effects from only the topology of the turbulent Galactic magnetic field. Thus, the statistical properties of $p$, $\psi$, and $S$ mostly reflect the structure of the magnetic field. Nevertheless, we search for potential signatures of varying grain alignment and dust properties. First, we analyse the product map $S \times p$, looking for residual trends. While $p$ decreases by a factor of 3--4 between $N_H=10^{20}$ cm$^{-2}$ and $N_H=2\times 10^{22}$ cm$^{-2}$, $S \times p$ decreases by only about 25%, a systematic trend observed in both the diffuse ISM and molecular clouds. Second, we find no systematic trend of $S \times p$ with the dust temperature, even though in the diffuse ISM lines of sight with high $p$ and low $S$ tend to have colder dust. We also compare Planck data with starlight polarization in the visible at high latitudes. The agreement in polarization angles is remarkable. Two polarization emission-to-extinction ratios that characterize dust optical properties depend only weakly on $N_H$ and converge towards the values previously determined for translucent lines of sight. We determine an upper limit for the polarization fraction in extinction of 13%, compatible with the $p_\mathrm{max}$ observed in emission. These results provide strong constraints for models of Galactic dust in diffuse gas.