Modeling the Milky Way Circumnuclear Disk: Rosettes and Rings

TL;DR

This study explores various orbital models of the Milky Way's circumnuclear disk, finding that a specific circular disk model best matches observed velocity maps, improving understanding of the region's gas dynamics.

Contribution

It introduces a comprehensive modeling approach considering non-Keplerian rosette orbits and multiple parameters, identifying the best-fit disk configuration for the CND.

Findings

Best-fit model is a circular disk with inner radius 1.0 pc and outer radius 2.9 pc.

Inclination of the disk is 60 degrees, with a position angle of 35 degrees.

Model comparison with observations supports the circular disk structure as most accurate.

Abstract

The Milky Way Galactic Center hosts a $\sim4\times10^{6}\,M_\odot$ supermassive black hole (SMBH), 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/ring (CND), 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 $\text{PA} = 35^{\circ}.$