Micro-arcsec Astrometry of the Milky Way and Beyond

Recently we have been able to achieve parallaxes and proper motions with accuracies approaching microarcseconds using the VLBA. The apparent proper motion of Sgr A* has been measured with extremely high accuracy, strongly constraining Θ0/R0 and potentially offering a new dynamical definition of the Galactic plane. The intrinsic motion of Sgr A* perpendicular to the Galactic plane is extremely small (∼ 1 km s) and comparable to that expected for a supermassive black hole. When coupled with IR observations of stellar orbits, this provides overwhelming evidence for the existence of supermassive black holes. We have measured trigonometric parallaxes and proper motions for about a dozen high-mass star forming regions (HMSFRs). Generally, kinematic distances exceed the true distances, suggesting that the IAU Galactic parameters, R0 and Θ0, may need modification. Values of the Solar Motion from our data agree with those obtained from Hipparcos data, but HMSFRs appear to be rotating slower than the Galaxy. Finally, the extraordinary astrometric capabilities of the VLBA have been used to measure extragalactic proper motions and to map masers in distant AGN accretion disks, which will yield direct estimates of H0.