Planck intermediate results. XXI. Comparison of polarized thermal emission from Galactic dust at 353GHz with optical interstellar polarization

The Planck survey provides unprecedented full-sky coverage of the submillimetre polarized emission from Galactic dust. In addition to the information on the direction of the Galactic magnetic field, this also brings new constraints on the properties of dust. The dust grains that emit the radiation seen by Planck in the submillimetre also extinguish and polarize starlight in the optical. Comparison of the polarization of the emission and of the interstellar polarization on selected lines of sight probed by stars provides unique new diagnostics of the emission and light scattering properties of dust, and therefore ultimately of the important model parameters, composition, size, shape, and alignment. Using ancillary catalogues of interstellar polarization and extinction of starlight, we obtain the degree of polarization, pV , and the optical depth in the V band to the star, V . We extract the submillimetre polarized intensity, PS, and total intensity, IS, measured toward these stars in the Planck 353 GHz channel. We compare the polarization direction (position angle) measured in the optical with that measured at 353 GHz, and compare the column density measure E(B V) with that inferred from the Planck product map of the submillimetre dust optical depth. For those lines of sight with little CO integrated intensity, as well as having polarization directions close to orthogonal and comparable values of the estimated column density, we correlate the projected polarization e ciencies in emission, QS=IS and US=IS, with those in extinction, qV= V and uV= V , to measure the polarization ratio RS=V = (PS=IS)=(pV= V ). We find a polarization ratio RS=V = 4:3 with statistical and systematic uncertainties 0:2 and 0:4, respectively. We focus directly on the polarization properties of the aligned grain population alone via an alternative ratio, the relative e ciency RP=p = PS=pV = 5:6 MJy sr 1 , with statistical and systematic uncertainties 0:2 and 0:4 MJy sr 1 , respectively. Our estimate of RS=V is reasonably compatible with predictions based on a range of current dust models that have been developed for the di use interstellar medium, not yet very discriminating among them. However, the observed RP=p is a more discriminating diagnostic for the polarizing grain population and is not compatible with predictions, the observations being higher by a factor of about 2.5. These new diagnostics from Planck, including the spectral dependence in the submillimetre, will be important for constraining and understanding the full complexity of the grain models, and for further interpretation of the Planck thermal dust polarization and refining the separation of this contamination of the cosmic microwave background.