The Rotation Curve of the Milky Way to 2.5 R/o From the Thickness of the HI Layer

A new approach to calculating the H I rotation curve of the outer Milky Way is presented. This method uses the variation with galactic longitude in the angular thickness of the H I layer to solve for the distance to slices through the H I data cube. The derived rotation curve shows excellent agreement with other kinematic constraints if the local galactic constants have values of R₀ = 7.9 +/- 0.8 kpc and Theta₀ = 200 +/- 10 km/s. This procedure also yields the variation in thickness of the H I layer with radius. If it is assumed that this gas has a constant velocity dispersion, then application of the hydrostatic equilibrium equation implies that the mass density in the plane of the Galaxy drops approximately exponentially with radius with an e-folding length of 0.49 +/- 0.06 R_0. Combining this information with the derived rotation curve then allows the construction of a full mass model for the Milky Way.