Modeling the Milky Way Circumnuclear Disk: Rosettes and Rings

TL;DR

This work tackles the dynamics of the Milky Way's circumnuclear disk (CND) near Sgr A* by embedding gas motions in the combined gravitational potential of the supermassive black hole and the nuclear star cluster. It systematically tests circular-disk and non-Keplerian rosette models, sampling ~3.3×10^5 configurations, and creates mock, beam-convolved ALMA-like velocity maps to compare against the observed HCN($J=1{-}0$) map. The key result is that a circular disk with inner radius $r_{ m in}=1.0$ pc, outer radius $r_{ m out}=2.9$ pc, inclination $i=60^ op$, and position angle ${ m PA}=35^ op$ best reproduces the global velocity structure, though rosette models can also match dispersion properties; SW-region residuals persist across models due to potentially non-gravitational processes. The study broadens CND modeling beyond single-ring assumptions, suggesting the CND is near-circularized, which has implications for its formation history and for interpreting gas inflow and star-formation potential in the GC. Future work should couple these static models to time-dependent simulations to capture possible self-intersections and non-gravitational effects observed in the data.

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 $\text{PA} = 35^{\circ}.$

Figures (3)

• Figure 1: Enclosed mass in the Milky Way's Galactic Center within radius $r$ from Sgr A*. The dashed line indicates the SMBH mass ($M_{\mathrm{SgrA*}}$), which does not change with distance. The dotted line indicates the stellar mass of the nuclear star cluster ($M_{\mathrm{NSC}}(r)$), obtained from Equations \ref{['eq:mass_dens_profile']} and \ref{['eq:M_nsc_encl']}Chatzopoulos_2015. The solid line is the sum of these components: $M_{\mathrm{enc}}(<r)$ = $M_{\mathrm{SgrA*}} + M_{\mathrm{NSC}}(r)$. The radius $r$ is marked in both pc (top) and arcsec (bottom).
• Figure 2: Emission-weighted mean velocity map (moment-1) of the CND in the HCN ($J=1{-}0$) line, constructed using observational data with signal-to-noise $\sigma \geq 20$ from Hsieh_2021. The data extends further than the plotted region, but we crop the map to retain only the most prominent features of the CND. The synthesized beam, which has a FWHM of $3.69" \times \,2.34"$ at a PA of $85.2\degree$, is depicted in the bottom left corner. The black star indicates the location of Sgr A*.
• Figure 3: Column 1 displays the mean velocity (moment-1) maps from each of our best-fit models. Column 2 overlays them with the observational velocity map (Figure \ref{['fig:obs_panel_fig']}). Column 3 plots the distance-weighted average of the velocity residuals between the observational and model maps (see Equation \ref{['eq:panelFig_residuals']}). Column 4 depicts these same residuals using normalized histograms, with their probability values multiplied by a factor of 100. The $\mathrm{x}{-}$axes and $\mathrm{y}{-}$axes in the first three columns indicate the $\mathrm{RA}$ and $\mathrm{dec}$ offset from Sgr A*, respectively. The bottom-left corner of these panels shows the kernel used to convolve the model data cubes, which is identical to the synthesized beam of the observational data Hsieh_2021. The $\mathrm{x}{-}$axes in column 4 are velocity residuals in units of $\mathrm{km\,s^{-1}}$, and the red dotted lines mark $0\,\mathrm{km\,s^{-1}}$.