We derive new constraints on the mass, rotation, orbit structure and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al. (2014), including 2'500 line-of-sight velocities and 10'000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions σ l and σ b cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 ′′ by a superposition of a spheroidal cluster with scale ∼ 100 ′′ and a much larger nuclear disk component. We compute the self-consistent two-integral distribution function f (E, L z ) for this density model, and add rotation self-consistently. We find that: (i) The orbit structure of the f (E, L z ) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v l -histograms. (ii) This requires an axial ratio near q 1 = 0.7 consistent with our determination from star counts, q 1 = 0.73 ± 0.04 for r < 70 ′′ . (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M * (r < 100 ′′ ) = (8.94±0.31| stat ±0.9| syst )×10 6 M ⊙ , M • = (3.86±0.14| stat ±0.4| syst )×10 6 M ⊙ , and R 0 = 8.27±0.09| stat ±0.1| syst kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modeling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A * strongly reduces the degeneracy between black hole mass and Galactic centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al. (2009) gives M • = (4.23±0.14)×10 6 M ⊙ and R 0 = 8.33±0.11 kpc.

Here we present the fundamental properties of the nuclear cluster of the Milky Way. First, we derive its structural properties by constructing a density map of the central 1000 using extinction-corrected star counts. We can describe the data with a two-component model built from Sersic profiles. The inner nearly spherical component is the nuclear cluster. The outer, strongly flattened component can be identified with the stellar component of the circumnuclear zone. Second, we enlarge the radius inside which detailed dynamics are available from 1 pc to 4 pc. We use more than 10000 individual proper motions and more than 2700 radial velocities. We determine the cluster mass by means of isotropic spherical Jeans modeling. We get a nuclear cluster mass within 100 of M 100 = (6.11 ± 0.52| fi xR 0 ± 0.97|R 0 ) × 10 6 M , which corresponds to a total cluster mass of MNC = (13.08 ± 2.51| fi xR 0 ± 2.08|R 0 ) × 10 6 M . By combination of our mass with the flux we calculate M/L = 0.50 ± 0.12M /L ,K s for the central 100 . That is broadly consistent with a Chabrier IMF. With its mass and a luminosity of MK s = −15.30±0.26 the nuclear cluster is a bright and massive specimen with a typical size.

In general relativity, the angular radius of the shadow of a black hole is primarily determined by its mass-to-distance ratio and depends only weakly on its spin and inclination. If general relativity is violated, however, the shadow size may also depend strongly on parametric deviations from the Kerr metric. Based on a reconstructed image of Sagittarius A * (Sgr A * ) from a simulated one-day observing run of a seven-station Event Horizon Telescope (EHT) array, we employ a Markov chain Monte Carlo algorithm to demonstrate that such an observation can measure the angular radius of the shadow of Sgr A * with an uncertainty of ∼ 1.5 µas (6%). We show that existing mass and distance measurements can be improved significantly when combined with upcoming EHT measurements of the shadow size and that tight constraints on potential deviations from the Kerr metric can be obtained.

The mean absolute extinction towards the central parsec of the Milky Way is A K 3 mag, including both foreground and Galactic center dust. Here we present a measurement of dust extinction within the Galactic old nuclear star cluster (NSC), based on combining differential extinctions of NSC stars with their υ l proper motions along Galactic longitude. Extinction within the NSC preferentially affects stars at its far side, and because the NSC rotates, this causes higher extinctions for NSC stars with negative υ l , as well as an asymmetry in the υ l -histograms. We model these effects using an axisymmetric dynamical model of the NSC in combination with simple models for the dust distribution. Comparing the predicted asymmetry to data for ∼ 7 100 stars in several NSC fields, we find that dust associated with the Galactic center mini-spiral with extinction A K 0.15 − 0.8 mag explains most of the data. The largest extinction A K 0.8 mag is found in the region of the Western arm of the mini-spiral. Comparing with total A K determined from stellar colors, we determine the extinction in front of the NSC. Finally, we estimate that for a typical extinction of A K 0.4 the statistical parallax of the NSC changes by ∼ 0.4%.

We report the first results from our program to examine the metallicity distribution of the Milky Way nuclear star cluster connected to SgrA*, with the goal of inferring the star formation and enrichment history of this system, as well as its connection and relationship with the central 100 pc of the bulge/bar system. We present the first high resolution (R∼ 24, 000), detailed abundance analysis of a K = 10.2 metal-poor, alpha-enhanced red giant projected at 1.5 pc from the Galactic Center, using NIRSPEC on Keck II. A careful analysis of the dynamics and color of the star locates it at about 26 +54 −16 pc line-of-sight distance in front of the nuclear cluster. It probably belongs to one of the nuclear components (cluster or disk), not to the bar-bulge or classical disk. A detailed spectroscopic synthesis, using a new linelist in the K band, finds [Fe/H]∼ −1.0 and [α/Fe]∼ +0.4, consistent with stars of similar metallicity in the bulge. As known giants with comparable [Fe/H] and alpha enhancement are old, we conclude that this star is most likely to be a representative of the ∼ 10 Gyr old population. This is also the most metal poor confirmed red giant yet discovered in vicinity of the nuclear cluster of the Galactic Center. We consider recent reports in the literature of a surprisingly large number of metal poor giants in the Galactic Center, but the reported gravities of log g ∼ 4 for these stars calls into question their reported metallicities.