A CO survey of the southern Milky Way - The mean radial distribution of molecular clouds within the solar circle

Abstract

The first out-of-plane CO survey of the southern Milky Way has been completed using the Columbia 1.2 m Millimeter-Wave Telescope at Cerro Tololo, Chile, and combined with the Northern CO Survey made with the Columbia Telescope in New York City to provide homogeneous coverage of the inner Galaxy. From these data we derived the mean radial distribution of molecular clouds in the Galactic disk for R = 2-10 kpc. About 70% of the molecular gas lies in a well-defined ring, with an inner radius of 4 kpc and an outer radius of 8 kpc. A total H2 mass of 1.2 x 10 9 Msun has been found for R = 2-10 kpc. A separate analysis of the Northern and Southern data shows that, although the mean radius of the distribution (6.5 kpc) and the mean thickness of the molecular disk (70 pc HWHM) are roughly constant from north to south, the radial dependence of the H2 density distribution changes enough to imply large-scale deviations from azimuthal symmetry. The Stony Brook/Massachusetts Northern CO survey fit to an axisymmetric model of the molecular Galaxy yields a value of the total H2 mass 2.2 times larger than ours. This discrepancy has been resolved into three comparable factors: different instrumental calibrations (20%); different proportionality constants to convert CO luminosities into H2 column densities (30%); and different statistical analyses (40%). Of these two axisymmetric analyses, only ours is self-consistent in that it can reproduce the observed longitudinal distribution of CO intensity integrated in velocity and Galactic latitude, I(I). When compared to the H I mass (0.9 x 10 9 Msun), the present value of the H2 mass, which is largely free of systematic instrumental errors because the N(H2)/W(CO) ratio used here was calibrated directly from Columbia CO data, implies a rough equipartition between atomic and molecular hydrogen in the Galactic disk within the solar circle.