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The Dynamical State of Filamentary Infrared Dark Clouds

The dense, cold gas of Infrared Dark Clouds (IRDCs) is thought to be representative of the initial conditions of massive star and star cluster formation. We analyze 13CO(J=1-0) line emission data from the Galactic Ring Survey of Jackson et al. for two filamentary IRDCs, comparing the mass surface densities derived from 13CO, Sigma_13CO, with those derived from mid-infrared small median filter extinction mapping, Sigma_SMF, by Butler & Tan. After accounting for molecular envelopes around the filaments, we find approximately linear relations between Sigma_CO and Sigma_SMF, i.e. an approximately constant ratio Sigma_CO/Sigma_SMF in the clouds. There is a variation of about a factor of two between the two clouds. We find evidence for a modest decrease of Sigma_CO/Sigma_SMF with increasing mass surface density, which may be due to a systematic decrease in temperature, increase in importance of high 13CO opacity cores, increase in dust opacity, or decrease in 13CO abundance due to depletion in regions of higher column density. We perform ellipsoidal and filamentary virial analyses of the clouds, finding that the surface pressure terms are dynamically important and that globally the filaments may not yet have reached virial equilibrium. Some local regions along the filaments appear to be close to virial equilibrium, although still with dynamically important surface pressures, and these appear to be sites where star formation is most active.