The nuclear star cluster of the Milky Way

TL;DR

This study provides detailed insights into the structure, stellar populations, and dynamics of the Milky Way's nuclear star cluster, highlighting its density profile, isotropy, and the mass distribution near Sagittarius A* using high-resolution observations.

Contribution

It offers the first detailed kinematic and structural analysis of the Milky Way's nuclear star cluster, including proper motions and mass modeling near Sagittarius A*.

Findings

Density profile follows a power-law with index ~1.8 beyond 1 pc

Proper motions for over 6000 stars within 1 pc are presented

Extended stellar mass of 0.5-2 million solar masses is confirmed in the central parsec

Abstract

The nuclear star cluster of the Milky Way is a unique target in the Universe. Contrary to extragalactic nuclear star clusters, using current technology it can be resolved into tens of thousands of individual stars. This allows us to study in detail its spatial and velocity structure as well as the different stellar populations that make up the cluster. Moreover, the Milky Way is one of the very few cases where we have firm evidence for the co-existence of a nuclear star cluster with a central supermassive black hole, Sagittarius A*. The number density of stars in the Galactic center nuclear star cluster can be well described, at distances $\gtrsim1$ pc from Sagittarius A*, by a power-law of the form $\rho(r)\propto r^{-\gamma}$ with an index of $\gamma\approx1.8$. In the central parsec the index of the power-law becomes much flatter and decreases to $\gamma\approx1.2$. We present proper motions for more than 6000 stars within 1 pc in projection from the central black hole. The cluster appears isotropic at projected distances $\gtrsim0.5$ pc from Sagittarius A*. Outside of 0.5 pc and out to 1.0 pc the velocity dispersion appears to stay constant. A robust result of our Jeans modeling of the data is the required presence of $0.5-2.0\times10^{6} M_{\odot}$ of extended (stellar) mass in the central parsec of the Galaxy.