The Nuclear Star Cluster


Our long-term monitoring of the Galactic Center region with NACO and SINFONI has led to a very valuable data base of stellar motions: we have measured proper motions for more than 10,000 stars and radial velocities for almost 3,000 stars (Fritz et al. 2016). We can determine the dynamic configuration of the nuclear cluster together with the density profile (Chatzopoulos et al. 2015). This allows us to measure the mass of the central point source, the distance to the system, and the mass profile of the extended cluster completely independently from the stellar orbits. The mass of and distance to Sgr A* are consistent with our results from the stellar orbits, although since the advent of GRAVITY, they are not competitive anymore.


Using the kinematic data together with the stellar number surface density profile, we can determine the dynamic configuration of the nuclear cluster (Chatzopoulos et al. 2015) and the mass profile. The latter shows that the massive black hole dominates the mass in the central parsec.





At smaller radii, the mass profile can be determined from the shape of the stellar orbits and the mass they enclose. For stars on close orbits, we have not yet detected any deviation in the shape of the orbits from what general relativity predicts. Stars at larger radii only show a marginal increase of the enclosed mass at around 3 arcseconds distance, corresponding to 25,000 astronomical units or 0.1 parsecs. Hence, throughout the central 0.1 parsecs down to the size of the pericenter of the star passing closest to Sgr A* (S29), at 100 astronomical units, the mass of the nuclear cluster is not detectable anymore due to the dominance of Sgr A*’s gravity.

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