Magnetically Dominated Parallel Interstellar Filaments at the Infrared
  Dark Cloud G14.225-0.506
  The G14.225-0.506 infrared dark cloud (IRDC G14.2) displays a remarkable
complex of parallel dense molecular filaments projected on the plane of the
sky. Previous dust emission and molecular-line studies have speculated whether
magnetic fields could have played an important role in the formation of such
long-shaped structures, which are hosts to numerous young stellar sources. In
this work we have conducted a vast polarimetric survey at optical and
near-infrared wavelengths in order to study the morphology of magnetic field
lines in IRDC G14.2 through the observation of background stars. The
orientation of interstellar polarization, which traces magnetic field lines, is
perpendicular to most of the filamentary features within the cloud.
Additionally, the larger-scale molecular cloud as a whole exhibits an elongated
shape also perpendicular to magnetic fields. Estimates of magnetic field
strengths indicate values in the range $320 - 550\,\mu$G, which allows
sub-alfv\'enic conditions, but does not prevent the gravitational collapse of
hub-filament structures, which in general are close to the critical state.
These characteristics suggest that magnetic fields played the main role in
regulating the collapse from large to small scales, leading to the formation of
series of parallel elongated structures. The morphology is also consistent with
numerical simulations that show how gravitational instabilities develop under
strong magnetic fields. Finally, the results corroborate the hypothesis that a
strong support from internal magnetic fields might explain why the cloud seems
to be contracting on a time scale 2-3 times larger than what is expected from a
free-fall collapse.
