Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust

TL;DR

Planck's 353 GHz polarization survey delivers the first all-sky view of polarized thermal dust emission, enabling a detailed probe of the Galactic magnetic field geometry and dust grain alignment. Using a Bayesian framework that leverages the full noise covariance and careful BPM corrections, the authors produce reliable maps of polarization fraction and angle, along with the polarization angle dispersion function $\Delta\psi$. They find $p$ ranges from near 0 to about $2\times 10^{1}\%$, with a conservative $p_{\max}=19.8\%$, and a pronounced decline of $p$ with increasing column density, driven by both radiative alignment and magnetic-field tangling along the line of sight. The $\Delta\psi$ structures, which anti-correlate with $p$, indicate pervasive, filamentary field-rotation effects; comparisons with synchrotron data and RM measurements reveal large-scale agreement but also significant LOS differences, underscoring the three-dimensional complexity of the Galactic magnetic field and the value of dust polarization as a complementary tracer for GMF studies.

Abstract

This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium.We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

Figures (26)

• Figure 1: Planck 353 $\rm GHz$ polarization maps at $1^\circ$ resolution. Upper: $Q$ Stokes parameter map. Lower: $U$ Stokes parameter map. The maps are shown with the same colour scale. High values are saturated to enhance mid-latitude structures. The values shown have been bias corrected as described in Sect. \ref{['sec:polarparam']}. These maps, as well as those in following figures, are shown in Galactic coordinates with the galactic center in the middle and longitude increasing to the left. The data is masked as described in Sect.\ref{['sec:othercorrections']}.
• Figure 2: Planck 353 $\rm GHz$ polarized intensity ($P$) map at $1^\circ$ resolution in log$_{10}$ scale. The values shown have been bias corrected as described in Sect. \ref{['sec:polarparam']}. The same mask as in Fig.\ref{['fig:rawpolarmaps']} is applied. The full sky map of the unpolarized intensity $I$ entering the calculation of $P$ is shown in Fig. \ref{['fig:polar_psi_and_sigpsi']}.
• Figure 3: Histograms of the observed polarized angle at the full data resolution towards the Galactic plane ($|b_{\rm II}|<5\degr$) for the four Galactic quadrants. The various curves show data uncorrected for bandpass mismatch (red), and corrected using sky coupling coefficients derived either from ground (method A: green) or sky measurements (method B: dark blue). The vertical dashed lines show the peak value obtained from fitting the histograms with a Gaussian.
• Figure 4: Upper: Map of the 353 $\rm GHz$ polarization fraction $p$ at $1^\circ$ resolution. The colour scale is linear and ranges from 0% to 20%. Lower: Map of the 353 $\rm GHz$ polarization fraction uncertainty, $\sigma_{p}$, at $1^\circ$ resolution in log$_{10}$ scale. The colour scale is from $\sigma_{p}=0.1\%$ to $\sigma_{p}=10\%$. The data are not shown in the grey areas where the dust emission is not dominant or where residuals were identified comparing individual surveys (see Sect.\ref{['sec:othercorrections']}). The polarization fraction is obtained using the Bayesian method with a mean posterior estimator (see Sect.\ref{['sec:polarparam']}). The uncertainty map includes statistical and systematic contributions. The same mask as in Fig.\ref{['fig:rawpolarmaps']} is applied.
• Figure 5: Upper: Map of the apparent magnetic field ($\langle\vec{B}_\perp\rangle$) orientation. The normalized lines were obtained by rotating the measured 353 $\rm GHz$ polarization angles by $90^\circ$. The length of the polarization vectors is fixed and does not reflect polarization fraction. The colour map shows the 353 $\rm GHz$ emission in log$_{10}$ scale and ranges from $10^{-2}$ to $10\rm MJysr^{-1}$ MJysr^-1$$. Lower: Map of the 353 $\rm GHz$ polarization angle uncertainty ($\sigma_{\psi}$) at $1^\circ$ resolution. The scale is linear from $\sigma_{\psi}=0\degr$ to $\sigma_{\psi}=52.3\degr$. The polarization angle is obtained using the Bayesian method with a mean posterior estimator (see Sect.\ref{['sec:polarparam']}). The uncertainty map includes statistical and systematic contributions. The same mask as in Fig.\ref{['fig:rawpolarmaps']} is applied.