Planck intermediate results: XXXIII. Signature of the magnetic field geometry of interstellar filaments in dust polarization maps

Planck observations at 353?GHz provide the first fully sampled maps of the polarized dust emission towards interstellar filaments and their backgrounds. We present the polarization maps of three nearby (several parsecs long) star-forming filaments of moderate column density: Musca, B211, and L1506. These three filaments are detected above the background in dust total and polarized emission. We recognize the degeneracy between the dust alignment efficiency (by, e.g., radiative torques) and the structure of the B-field in causing variations in p, but we argue that the decrease in p from the backgrounds to the filaments results in part from depolarization associated with the 3D structure of the B-field: both its orientation in the POS and with respect to the POS. They show that both the mean field and its fluctuations in the filaments are different from those of their backgrounds, which points to a coupling between the matter and the B-field in the filament formation process.