Modeling the Milky Way Circumnuclear Disk: Rosettes and Rings

Abstract

The Milky Way Galactic Center hosts a $\sim4\times10^{6}\,M_\odot$ supermassive black hole (SMBH), Sagittarius A* (Sgr A*). The dominant structures in its immediate vicinity are the nuclear star cluster (NSC), whose enclosed mass at 2 pc is approximately half that of the SMBH, and the circumnuclear disk (CND)/ring, which extends between $\sim0.5$ pc and $\sim3$ pc from Sgr A* and is the largest reservoir of molecular gas in this region. Existing models of the CND commonly use one circular orbit to describe the motion of its gas. Here, we explore a much broader range of models. In the combined potential of Sgr A* and the NSC, we consider non-Keplerian rosette orbits as well as a circular disk, which is formed using a finely spaced set of concentric rings. For both systems, we test various inner/outer radii, inclinations, and position angles, sampling a total of $\sim3.3 \times 10^{5}$ models. We then conduct mock observations of all models to construct velocity maps, which we compare with HCN ($J=1{-}0$) observations of the CND. We find that the best-fitting model is a circular disk with inner and outer radii of 1.0 pc and 2.9 pc, an inclination of $i=60^{\circ}$, and a position angle of $\text{PA} = 35^{\circ}.$