ON THE ROTATION SPEED OF THE MILKY WAY DETERMINED FROM H i EMISSION

The circular rotation speed of the Milky Way at the solar radius, Θ 0 , has been estimated to be 220 km s −1 by fitting the maximum velocity of H i emission as a function of Galactic longitude. This result is in tension with a recent estimate of Θ 0 = 240 km s −1 , based on Very Long Baseline Interferometry (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 Θ 0 from the H i data when a flat rotation curve is assumed. This relieves the tension between the methods and favors Θ 0 = 240 km s −1 .