A spectroscopic map of the Galactic centre: Integrated light and dynamical modelling

Abstract

The centre of the Milky Way is occupied by a nuclear star cluster that contains the supermassive black hole Sgr A*. The cluster is embedded in the larger surrounding nuclear stellar disc. These three components dominate the mass budget of the Galactic centre at different radial scales. The mass distribution of the Galactic centre has been studied extensively using observations of individual bright stars and various dynamical modelling approaches. The situation differs for external galaxies, where observations are often limited to the integrated line-of-sight kinematics. For such systems, triaxial orbit-based dynamical modelling has become a standard method of deriving mass distributions and stellar orbit distributions. We aim to apply and test this method on the Galactic centre. We extracted stellar line-of-sight kinematic maps of the inner ~3 pc x 66 pc region of the Galactic centre. We used the DYNAMITE code, which calculates an orbit library in a given gravitational potential and computes model kinematic maps. These maps were then compared to the observed kinematic maps, and the gravitational potential and orbit distribution of the Galactic centre were constrained. We recover the correct mass of Sgr A*, and our stellar mass distributions are in agreement with the literature, albeit with larger uncertainties. The stellar structures are at most mildly triaxial and close to oblate. The stellar orbit distribution in the inner region is dominated by dynamically warm and hot orbits. At larger scales, dynamically cold -- highly rotating -- orbits have the largest weights. The dominance of hot and warm orbits is a consequence of short dynamical timescales in the inner Galactic centre, causing dynamical heating. The presence of cold orbits at large radii may be explained by the longer heating timescales in this region, and by the stars in the outer nuclear stellar disc being younger.[abridged]