The Vertical Equilibrium of Molecular Gas in the Galactic Disk

Abstract

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 ΔR ~ 200-400 pc; and is parametrized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion. This model is then fit to the $^{12}CO$ survey of Knapp et al.(1985) to determine the best fit parameters. The reduced χ^2 from fitting the models (considering only the photon statistics) range from 2 to 19. The main source of error is the `shot noise' due to the small number of clouds. Simulations using discrete molecular clouds are carried out to estimate the errors. The scale height of the molecular gas increases with radius. The velocity dispersion varies from ~ 2 km/s to 11 km/s with a typical uncertainty of ~3 km/s and shows a monotonic increase with galactic radius. The midplane mass density of the disk calculated from the scale-height and velocity dispersion is consistent with the local value ρ_0(R_0) of 0.2 \msun pc^{-3}.