The Vertical Equilibrium of the Molecular Gas in the Galaxy

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Session 60 -- Galactic Structure
Oral presentation, Thursday, January 13, 10:15-11:45, Crystal Forum Room (Crystal City Marriott)

[60.03] The Vertical Equilibrium of the Molecular Gas in the Galaxy

S. Malhotra (Princeton University Observatory)

\def\kms{{\rm\,km\,s^{-1}}} \def\pc{{\rm\,pc}}

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. These quantities are determined for the tangent point gas in a survey of $^{12}CO(1\rightarrow0)$ in the first quadrant.

The tangent point emission is modeled in two dimensions (velocity and galactic latitude), taking into account the velocity dispersion of the gas and velocity crowding at the tangent points, and the subsequent superposition of emission from a large path length near the tangent point. The model is parametrized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion.

The terminal velocities obtained by fitting this model are in good agreement with those obtained from the HI data. The velocity dispersion varies between $\sim 2 \kms$ to $11 \kms$ and is consistent with a monotonic increase with galactic radius. The scale height also increases with radius. The z-location of the midplane shows a smooth undulation between the longitudes $25 \deg$ and $\ 60 \deg$. The maximum deviation from the plane is $ \simeq -50 \pc $, comparable to the scale height of the molecular gas.

To estimate the uncertainties introduced by the cloudy medium we simulate the molecular cloud population in the galaxy, and it's `observation' from our perspective. Using the standard cloud properties and radial distribution in the galaxy, we get values of $\chi^2$ per degree of freedom similar to those obtained from fitting the models to the observations.

For most lines of sight where the latitude coverage of the data is greater than 3 scale heights, a higher than expected mass of gas is found at high altitudes ($z > 3\sigma_z$). These high altitude clouds are fainter and smaller than the giant molecular clouds. These data suggest a population of lower mass clouds with a significantly larger scale height than the giant molecular clouds.

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