An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis

• Boehle, A.
;
• Ghez, A. M.
;
• Schödel, R.
;
• Meyer, L.
;
• Yelda, S.
;
• Albers, S.
;
• Martinez, G. D.
;
• Becklin, E. E.
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• Do, T.
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• Lu, J. R.
;
• Matthews, K.
;
• Morris, M. R.
;
• Sitarski, B.
;
• Witzel, G.

Abstract

We present new, more precise measurements of the mass and distance of our Galaxy’s central supermassive black hole, Sgr A*. These results stem from a new analysis that more than doubles the time baseline for astrometry of faint stars orbiting Sgr A*, combining 2 decades of speckle imaging and adaptive optics data. Specifically, we improve our analysis of the speckle images by using information about a star’s orbit from the deep adaptive optics data (2005-2013) to inform the search for the star in the speckle years (1995-2005). When this new analysis technique is combined with the first complete re-reduction of Keck Galactic Center speckle images using speckle holography, we are able to track the short-period star S0-38 (K-band magnitude = 17, orbital period = 19 yr) through the speckle years. We use the kinematic measurements from speckle holography and adaptive optics to estimate the orbits of S0-38 and S0-2 and thereby improve our constraints of the mass (M _bh) and distance (R _o ) of Sgr A*: M _bh = (4.02 ± 0.16 ± 0.04) × 10⁶ M _⊙ and 7.86 ± 0.14 ± 0.04 kpc. The uncertainties in M _bh and R _o as determined by the combined orbital fit of S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, compared to an orbital fit of S0-2 alone and a factor of ∼2.5 compared to previous results from stellar orbits. This analysis also limits the extended dark mass within 0.01 pc to less than 0.13 × 10⁶ M _⊙ at 99.7% confidence, a factor of 3 lower compared to prior work.