Abstract
To understand the physical conditions for filament formation, the magnetic field measurements would provide crucial information. Here we present our optical and infrared polarization observations toward IC5146 taken with AIMPOL (India), TRIPOL (Taiwan) and Mimir (US). IC5146 is one of the filamentary clouds observed in Herschel Gould Belt Survey, and Arzoumanian et al. (2011) claimed that the complex network of filaments discovered within the cloud favors the scenario that the filaments network are generated by large scale MHD turbulence and fragment into prestellar cores by gravitational instability. Our results reveal that the large scale structure of magnetic field is well perpendicular to the main filament, but more or less parallel to the sub-filaments, which are structure extended out from the main filaments. We have also conducted CO observations which show that the material in the sub-filament is flowing to the main-filament along the magnetic field; this result suggests the gas is possibly confined by magnetic field. In addition, the magnetic field strength map derived from the Chandrasekhar-Fermi method indicates a smooth magnetic enhancement with a scaling factor of ~0.5, which suggest the gas is supported by the magnetic field while it is collapsing. Furthermore, the magnetic strength and density reveals that the cloud is evolving from magnetically subcritical in diffuse region to supercritical in dense region, emphasizing the importance of the question how the filament can remove the magnetic flux during its collapsing. Our results suggest that magnetic fields are one of the key factor during the filament formation and fragmentation processes.