μas VLBI Astrometry

Over the past decade, we have developed and tested the techniques needed to achieve better than 30 micro-arcseconds relative positional accuracy between maser sources and background QSOs at each epoch. 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 in 2009.

Galactic Structure, Fundamental Parameters, and Noncircular Motions With the current data, we already locate the Perseus spiral arm and the local arm. Combining positions, distances, proper motions, and radial velocities yields complete three-dimensional kinematic information. We find that star-forming regions on average are orbiting the Galaxy ~15 km/s slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center and a circular rotation speed: R0 = 8.4 ± 0.6 kpc Θo = 254 ± 16 km/s. The ratio Θo/Ro can be determined to higher accuracy than either parameter individually, and we find it to be 30.3 ± 0.9 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 or slightly rising with Galactocentric distance. 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. We offer a "revised" prescription for calculating kinematic distances and their uncertainties, as well as a new approach for defining Galactic coordinates. 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. (Published in Reid et al. 2009 ApJ 700, 137; see also NRAO or CfA press releases)

Milky Way (Illustration Credit: Robert Hurt, IPAC; Mark Reid, CfA, NRAO/AUI/NSF)