2012ApJ...756L..13H

The initial conditions of massive star and star cluster formation are expected to be cold, dense, and high column density regions of the interstellar medium, which can reveal themselves via near-, mid-, and even far-infrared absorption as infrared dark clouds (IRDCs). Elucidating the dynamical state of IRDCs thus constrains theoretical models of these complex processes. In particular, it is important to assess whether IRDCs have reached virial equilibrium, where the internal pressure balances that due to the self-gravitating weight of the cloud plus the pressure of the external environmental. We study this question for the filamentary IRDC G035.39-00.33 by deriving mass from combined NIR and MIR extinction maps and velocity dispersion from C¹⁸O (1-0) and (2-1) line emission. In contrast to our previous moderately super-virial results based on ¹³CO emission and MIR-only extinction mapping, with improved mass measurements we now find that the filament is consistent with being in virial equilibrium, at least in its central parsec-wide region where ∼1000 M_☉ snakes along several parsecs. This equilibrium state does not require large-scale net support or confinement by magnetic fields.