Multiwavelength Polarimetry of the Filamentary Cloud IC5146: II. Magnetic Field Structures

The IC5146 cloud is a nearby star-forming region in Cygnus, consisting of molecular gas filaments in a variety of evolutionary stages. We used optical and near-infrared polarization data toward the IC5146 cloud, reported in the first paper of this series, to reveal the magnetic fields in this cloud. Using the newly released $Gaia$ data, we found that the IC5146 cloud may contain two separate clouds: a first cloud, including the densest main filament at a distance of $\sim$600 pc, and a second cloud, associated with the Cocoon Nebula at a distance of $\sim$800 pc. The spatially averaged H-band polarization map revealed a well-ordered magnetic field morphology, with the polarization segments perpendicular to the main filament but parallel to the nearby sub-filaments, consistent with models assuming that the magnetic field is regulating cloud evolution. We estimated the magnetic field strength using the Davis-Chandrasekhar-Fermi method, and found that the magnetic field strength scales with volume density with a power-law index of $\sim0.5$ in the density range from $N_{H_2}\sim$ 10 to 3000 cm$^{-3}$, which indicates an anisotropic cloud contraction with a preferred direction along the magnetic field. In addition, the mass-to-flux ratio of the cloud gradually changes from subcritical to supercritical from the cloud envelope to the deep regions. These features are consistent with strong magnetic field star-formation models and suggest that the magnetic field is important in regulating the evolution of the IC5146 cloud.