A three-dimensional reconstruction of the interstellar magnetic field toward a star-forming region

Context. The polarized thermal emission from interstellar dust offers a valuable tool for probing both the dust and the magnetic field in the interstellar medium (ISM). However, existing observations only yield the total amount of dust emission along the line of sight (LoS), with no information on its LoS distribution. Aims. We present a new method designed to give access to the LoS distribution of the dust emission, both in terms of intensity and polarization. Methods. We relied on three kinematic gas tracers (HI, 12CO, and 13CO emission lines) to identify the different clouds present along the LoS. We decomposed the measured intensity of the dust emission, Id, into separate contributions from these clouds. We performed a similar decomposition of the measured Stokes parameters for linear polarization, Qd and Ud, to derive the polarization parameters of the different clouds, and from this we inferred the clouds’ magnetic field orientations. Results. We applied our method to a 3 deg2 region of the sky, centered on (l, b) = (139°30′, −3°16′) and exhibiting signs of star formation activity. We found this region to be dominated by an extended and bright cloud with nearly horizontal magnetic field, as expected from the nearly vertical polarization angles measured by Planck. More importantly, we detected the presence of two smaller, depolarizing molecular clouds with very different magnetic field orientations in the plane of the sky (≃65° and ≃45° from the horizontal). This is a novel and viable result, which cannot be directly read off the Planck polarization maps. Conclusions. The application of our method to the G139 region convincingly demonstrates the need to complement 2D polarization maps with 3D kinematic information when looking for reliable estimates of magnetic field orientations.