The vertical equilibrium of molecular gas in the galactic disk

We examine the vertical structure and equilibrium of the molecular gas layer in the galactic disk, measuring its scale height and velocity dispersion as a function of Galactic radius by modeling the CO emission at the tangent points. The model takes into account emission from a large path length along the line of sight, corresponding to an interval (∆R) of typically a few hundred parsecs in galactic radius; and is parametrized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion; these parameters are assumed to be constant over the interval ∆R. This model is then fit to the CO survey of Knapp et al.(1985) to determine the best fit parameters. The terminal velocity values are found to be in good agreement with those obtained from HI data. The χ per degree of freedom from fitting the models (considering only the photon statistics) range from 2 to 19, with a median value of 6. The main source of error is found to be the ‘shot noise’ due to the small number of clouds. Simulations of the tangent point emission using discrete molecular clouds are carried out to estimate the errors. We use the observed radial distribution of molecular gas and the ‘standard’ size-linewidth relation for molecular clouds in the simulations. Modeling the simulations gives values of χ per degree of freedom similar to those obtained from modeling the observations. The scale height of the distribution is found to increase fairly monotonically with radius. The value of the velocity dispersion varies from ∼ 2 km s to 11 km s with a typical uncertainty of ∼ 3 km s. The variation in velocity dispersion is consistent with a monotonic increase with galactic radius. The midplane mass density of the disk ρ0(R) is calculated from the scale-height and velocity dispersion (under the assumption that the velocity dispersion is isotropic) and is consistent with the local value ρ0(R0) of 0.2M⊙ pc −3 determined from stellar kinematics. Subject headings: interstellar gas, Galaxy, internal motions, mass models