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)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) to inform the search for the star in the speckle years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(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) ×10 6 M e 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×10 6 M e at 99.7% confidence, a factor of 3 lower compared to prior work.

Aims. The centre of the Milky Way is the nearest nucleus of a galaxy and offers a unique possibility to study the structure and dynamics of a dense stellar cluster around a super-massive black hole. Methods. We present high-resolution seeing limited and AO NIR imaging observations of the stellar cluster within about one parsec of Sgr A*, the massive black hole at the centre of the Milky Way. Stellar number counts and the diffuse background light density were extracted from these observations in order to examine the structure of the nuclear stellar cluster. A detailed map of the variation of interstellar extinction in the central ∼0.5 pc of the Milky Way is presented and used to correct the stellar number counts and diffuse light density. Results. Our findings are as follows: (a) a broken-power law provides an excellent fit to the overall structure of the GC nuclear cluster. The power-law slope of the cusp is Γ = 0.19 ± 0.05, the break radius is R break = 6.0 ± 1.0 or 0.22 ± 0.04 pc, and the cluster density decreases with a power-law index of Γ = 0.75 ± 0.1 outside of R break . (b) Using the best velocity dispersion measurements from the literature, we derive higher mass estimates for the central parsec than assumed until now. The inferred density of the cluster at the break radius is 2.8 ± 1.3 × 10 6 M pc −3 . This high density agrees well with the small extent and flat slope of the cusp. Possibly, the mass of the stars makes up only about 50% of the total cluster mass. (c) Possible indications of mass segregation in the cusp are found (d) The cluster appears not entirely homogeneous. Several density clumps are detected that are concentrated at projected distances of R = 3 and R = 7 from Sgr A*. (e) There appears to exist an under-density of horizontal branch/red clump stars near R = 5 , or an over-density of stars of similar brightness at R = 3 and R = 7 . (f) The extinction map in combination with cometary-like features in an L -band image may provide support for the assumption of an outflow from Sgr A*.

We present new kinematic measurements and modeling of a sample of 116 young stars in the central parsec of the Galaxy in order to investigate the properties of the young stellar disk. The measurements were derived from a combination of speckle and laser guide star adaptive optics imaging and integral field spectroscopy from the Keck telescopes. Compared to earlier disk studies, the most important kinematic measurement improvement is in the precision of the accelerations in the plane of the sky, which have a factor of six smaller uncertainties (σ ∼10 µas yr −2 ). We have also added the first radial velocity measurements for 8 young stars, increasing the sample at the largest radii (6 ′′ -12 ′′ ) by 25%. We derive the ensemble properties of the observed stars using Monte-Carlo simulations of mock data. There is one highly significant kinematic feature (∼20σ), corresponding to the well-known clockwise disk, and no significant feature is detected at the location of the previously claimed counterclockwise disk. The true disk fraction is estimated to be ∼20%, a factor of ∼2.5 lower than previous claims, suggesting that we may be observing the remnant of what used to be a more densely populated stellar disk. The similarity in the kinematic properties of the B stars and the O/WR stars suggests a common star formation event. The intrinsic eccentricity distribution of the disk stars is unimodal, with an average value of e =0.27±0.07, which we show can be achieved through dynamical relaxation in an initially circular disk with a moderately top-heavy mass function.