The role of magnetic fields in the formation of the filamentary infrared dark cloud G11.11-0.12

We report on the near-infrared polarimetric observations of G11.11-0.12 (hereafter G11) obtained with SIRPOL on the 1.4 m IRSF telescope. The starlight polarisation of the background stars reveals the on-sky component of magnetic fields in G11, and these are consistent with the field orientation observed from polarised dust emission at 850 mu m. The magnetic fields in G11 are perpendicular to the filament, and are independent of the filament's orientation relative to the Galactic plane. The field strength in the envelope of G11 is in the range 50 - 100 mu G, derived from two methods. The analyses of the magnetic fields and gas velocity dispersion indicate that the envelope of G11 is supersonic but sub-Alfv ' enic. The critical mass-to-flux ratio in the envelope of G11 is close to 1 and increases to greater than or similar to 1 on the spine of G11. The relative weights on the importance of magnetic fields, turbulence and gravity indicate that gravity dominates the dynamical state of G11, but with significant contribution from magnetic fields. The field strength, |B|, increases slower than the gas density, n, from the envelope to the spine of G11, characterized by |B|alpha n(0.3). The observed strength and orientation of magnetic fields in G11 imply that supersonic and sub-Alfvenic gas flow is channelled by the strong magnetic fields and is assembled into filaments perpendicular to the magnetic fields. The formation of low-mass stars is enhanced in the filaments with high column density, in agreement with the excess of low-mass protostars detected in the densest regions of G11.