Unified Rotation Curve of the Galaxy -- Decomposition into de Vaucouleurs Bulge, Disk, Dark Halo, and the 9-kpc Rotation Dip --

We present a unified rotation curve of the Galaxy re-constructed from existing data by re-calculating the distances and velocities for a set of Galactic constants, R₀ = 8 kpc and V₀ = 200km s^-1. We decomposed it into a bulge with de Vaucouleurs-law profile of half-mass scale radius 0.5kpc and mass 1.8 × 10¹⁰M_odot, an exponential disk of scale radius 3.5kpc and 6.5 × 10¹⁰M_odot, and an isothermal dark halo of terminal velocity 200km s^-1. A r^1/4-law fit was obtained for the first time for the Milky Way's rotation curve. After fitting by these fundamental structures, two local minima, or dips, of the rotation velocity are prominent at radii 3 and 9kpc. The 3-kpc dip is consistent with the observed bar. It is alternatively explained by a massive ring with the density maximum at a radius of 4kpc. The 9-kpc dip is clearly exhibited as the most peculiar feature in the Galactic rotation curve. We explain it by a massive ring of amplitude as large as 0.3 to 0.4-times the disk density with the density peak at a radius of 11kpc. This great ring may be related to the Perseus arm, while no peculiar feature of HI-gas is associated.