Triaxial orbit-based modelling of the Milky Way Nuclear Star Cluster

Feldmeier-Krause, A.; Zhu, L.; Neumayer, N.; van de Ven, G.; de Zeeuw, P. T.; Schödel, R.

April, 2017

We construct triaxial dynamical models for the Milky Way nuclear star cluster using Schwarzschild's orbit superposition technique. We fit the stellar kinematic maps presented in Feldmeier et al. The models are used to constrain the supermassive black hole mass M, dynamical mass-to-light ratio ϒ and the intrinsic shape of the cluster. Our best-fitting model has M = (3.0^{+1.1}_{-1.3}) × 106 M, ϒ = (0.90^{+0.76}_{-0.08}) M/L⊙, 4.5μmand a compression of the cluster along the line of sight. Our results are in agreement with the direct measurement of the supermassive black hole mass using the motion of stars on Keplerian orbits. The mass-to-light ratio is consistent with stellar population studies of other galaxies in the mid-infrared. It is possible that we underestimate M and overestimate the cluster's triaxiality due to observational effects. The spatially semiresolved kinematic data and extinction within the nuclear star cluster bias the observations to the near side of the cluster, and may appear as a compression of the nuclear star cluster along the line of sight. We derive a total dynamical mass for the Milky Way nuclear star cluster of MMWNSC = (2.1 ± 0.7) × 107 M within a sphere with radius r = 2 × reff = 8.4 pc. The best-fitting model is tangentially anisotropic in the central r = 0.5-2 pc of the nuclear star cluster, but close to isotropic at larger radii. Our triaxial models are able to recover complex kinematic substructures in the velocity map.