On the Rotation Speed of the Milky Way Determined from HI Emission

TL;DR

This paper compares different methods of estimating the Milky Way's rotation speed, finding that accounting for the rotation curve's curvature reconciles previous discrepancies and supports a higher value of 240 km/s.

Contribution

It demonstrates that the Milky Way's rotation curve curvature biases HI emission-based estimates, and incorporating this curvature aligns the results with VLBI measurements.

Findings

HI emission method underestimates Theta_o when assuming flat rotation curve

VLBI data suggests a slightly curved rotation curve

Accounting for curvature resolves tension between methods

Abstract

The circular rotation speed of the Milky Way at the solar radius, Theta_o, 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 Theta_o=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 Theta_o from the HI data when a flat rotation curve is assumed. This relieves the tension between the methods and favors Theta_o=240 km/s.