The rotation curve of the Galaxy has been measured to a galactocentric distance of 16 kpc by using CO observations of molecular complexes related to H II regions in the second and third quadrants. There is no evidence for a turnover in the velocity to the last measured point, and from R = 12 kpc to R = 16 kpc, the rotation curve appears to rise by about 30 km/s. The mass of the Galaxy interior to 16 kpc is about 300 billion solar masses, a value comparable to, but greater than, that of M31. The velocity dispersion of the complexes from the mean rotation curve is 8 km/s.
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Leo Blitz (1979) studied this question.