Dynamical mass distribution and velocity structure of the Galactic centre

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Wien, Austria
² Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

^★ Corresponding author: anja.krause@univie.ac.at

Received: 18 March 2025
Accepted: 23 May 2025

Abstract

Context. The inner ~200 pc region of the Milky Way contains a nuclear stellar disc and a nuclear star cluster that are embedded in the larger Galactic bar. These stellar systems overlap spatially, which makes it challenging to separate stars that belong to the nuclear stellar systems, to deduce their internal dynamics, and to derive the central Galactic potential.

Aims. Discrete stellar kinematics probe the mass distribution of a stellar system, and chemical tracers such as stellar metallicity can further separate multiple stellar populations that can have distinct kinematic properties. We took advantage of the information provided by discrete stellar kinematics and the metallicity of stars in the Galactic centre using discrete chemo-dynamical modelling.

Methods. We fitted axisymmetric Jeans models to discrete data of 4600 stars. We fitted the stars as either one population plus a background component or as two populations plus a background that represents the bar. In the one-population case, we tested the robustness of the inferred gravitational potential against a varying mass of the supermassive black hole, including dark matter, or a radially varying mass-to-light ratio.

Results. We obtained robust results on the stellar dynamical fit with a single population and a background component. We obtained a supermassive black hole mass of (4.35±0.24) × 10⁶ M_⊙, and we find that a dark matter component adds no more than a few percent to the total enclosed mass of the nuclear star cluster. The radial variation in the mass-to-light ratio is also negligible. We derived the enclosed mass profile of the inner ~60 pc of the Milky Way and found a lower mass than reported in the literature in the region of ~5–30 pc. In our two-population fit, we found a high-[M/H] population with a mild tangentially anisotropic velocity distribution and stronger rotational support than for the low-[M/H] population, which is radially anisotropic. The high-[M/H] population is dominant and contributes more than 90% to the total stellar density.

Conclusions. The properties of the high-[M/H] population are consistent with in situ formation after gas inflow from the Galactic disc via the bar. The distinct kinematic properties of the low-[M/H] population indicate a different origin.