THE ROTATION CURVE OF THE MILKY-WAY TO 2.5R0 FROM THE THICKNESS OF THE H I LAYER

We present a new approach to calculating the H I rotation curve of the outer Milky Way. 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 THETA-0 = 200 +/- 10 km s-1. This procedure also yields the variation in thickness of the H I layer with radius. If we assume 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 R0. Combining this information with the derived rotation curve then allows the construction of a full mass model for the Milky Way.