Micro-arcsecond VLBI Astrometry
Over the past decade, we have developed and tested the techniques that have enabled parallax measurements with accuracies of better than 10 micro-arcseconds. This meets or exceeds Gaia's parallax accuracy. While VLBI astrometry will not approach the number of parallaxes that Gaia will do, we can freely probe the entire Galactic plane, whereas Gaia will be severely limited there by extinction (e.g, Av=30 mag toward the Galactic center). Before beginning this project, we determined trigonometric parallaxes to 11 star forming regions with accuracies of order 10 micro-arcseconds. We also measured absolute proper motions with accuracies of order 1 km/s. These results have been published in a series of papers in the Astrophyscial Journal and Astronomy & Astrophysics since 2009.
Galactic Structure, Fundamental Parameters, and Noncircular Motions
With the current data, we already locate several spiral arms. Combining positions, distances, proper motions, and radial velocities yields complete three-dimensional kinematic information. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center and a circular rotation speed:
Θo = 236 ± 7 km/s.
The ratio Θo/Ro can be determined to higher accuracy than either parameter individually, and we find it to be 30.32 ± 0.27 km/s/kpc, in good agreement with the angular rotation rate determined from the proper motion of Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat between Galactocentric radii of 5 and 16 kpc. Kinematic distances are generally too large, sometimes by factors greater than 2; they can be brought into better agreement with the trigonometric parallaxes by increasing Ro from the IAU recommended value of 25.9 km/s/kpc to a value near 30 km/s/kpc. Finally, our estimates of Θo and Ro, when coupled with direct estimates of Ro, provide evidence that the rotation curve of the Milky Way is similar to that of the Andromeda galaxy, suggesting that the dark matter halos of these two dominant Local Group galaxy are comparably massive.
Milky Way with measured parallaxes (from Reid et al. 2019)