Planck intermediate results. LIV. Polarized dust foregrounds

The study of polarized dust emission has become entwined with the analysis of the cosmic microwave background (CMB) polarization. We use new Planckmaps to characterize Galactic dust emission as a foreground to the CMB polarization. We present PlanckEE, BB, and TE power spectra of dust polarization at 353 GHz for six nested skyregions covering from 24 to 71 % of the sky. We present power-law fits to the angular power spectra, yielding evidence for statistically significant variations of the exponents over skyregions and a difference between the values for the EE and BB spectra. The TE correlation and E/B power asymmetry extend to low multipoles that were not included in earlier Planckpolarization papers. We also report evidence for a positive TB dust signal. Combining data from Planckand WMAP, we determine the amplitudes and spectral energy distributions(SEDs) of polarized foregrounds, including the correlation between dust and synchrotron polarized emission, for the six skyregions as a function of multipole. This quantifies the challenge of the component separation procedure required for detecting the reionizationand recombination peaks of primordial CMB B modes. The SED of polarized dust emission is fit well by a single-temperature modified blackbody emission law from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean spectral indexfor dust polarization is $\beta_{\rm d}^{P} = 1.53\pm0.02 $. By fitting multi-frequency cross-spectra, we examine the correlation of the dust polarization maps across frequency. We find no evidence for decorrelation. If the Plancklimit for the largest skyregion applies to the smaller skyregions observed by sub-orbital experiments, then decorrelationmight not be a problem for CMB experiments aiming at a primordial B-mode detection limit on the tensor-to-scalar ratio $r\simeq0.01$ at the recombination peak.