On the Rotation Speed of the Milky Way Determined from HI Emission
Abstract
The circular rotation speed of the Milky Way at the solar radius, Thetao, has been estimated to be 220 km/s by fitting the maximum velocity of HI emission as a function of Galactic longitude. This result is in tension with a recent estimate of Thetao=240 km/s, based on VLBI parallaxes and proper motions from the BeSSeL and VERA surveys for large numbers of high-mass star forming regions across the Milky Way. We find that the rotation curve best fitted to the VLBI data is slightly curved, and that this curvature results in a biased estimate of Thetao from the HI data when a flat rotation curve is assumed. This relieves the tension between the methods and favors Thetao=240 km/s.
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