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.
