Probing the properties of the Milky Way's central supermassive black hole with stellar orbits

We report new precision measurements of the properties of our Galaxy's supermassive black hole. Based on astrometric (1995-2007) and radial velocity (2000-2007) measurements from the W. M. Keck 10 meter telescopes, the Keplerian orbital parameters for the short period star S0-2 imply a distance of 8.3 ± 0.3 kpc, an enclosed mass of 4.8 ± 0.3 × 10⁶M_⊙, and a black hole position that is localized to within ± 1 mas and that is consistent with the position of SgrA*-IR. Astrometric bias from source confusion is identified as a significant source of systematic error and is accounted for in this study. Our black hole mass and distance are significantly higher than previous estimates. The higher mass estimate brings the Galaxy into better agreement with the relationship between the mass of the central black hole and the velocity dispersion of the host galaxy's bulge observed for nearby galaxies. It also raises the orbital period of the innermost stable orbit of a non-spinning black hole to 38 min and increases the Rauch-Tremaine resonant relaxation timescales for stars in the vicinity of the central black hole. Taking the black hole's distance as a measure of R₀, which is a fundamental scale for our Galaxy, and other measurements of galactic constants, we infer a value of the Galaxy's local rotation speed (θ₀) of 255 ± 13 km s⁻¹. With the precisions of the astrometric and radial velocity measurements that are now possible with Laser Guide Star Adaptive Optics, we expect to be able to measure Ro to an accuracy of ~ 1% within the next ten years, which could considerably reduce the uncertainty in the cosmological distance ladder.