The Circumnuclear Disk Revealed by ALMA. I. Dense Clouds and Tides in the Galactic Center

Abstract: Utilizing the Atacama Large Millimeter/submillimeter Array (ALMA), we present CS line maps in five rotational lines ($J_{\rm u}=7, 5, 4, 3, 2$) toward the circumnuclear disk (CND) and streamers of the Galactic Center. Our primary goal is to resolve the compact structures within the CND and the streamers, in order to understand the stability conditions of molecular cores in the vicinity of the supermassive black hole (SMBH) Sgr A*. Our data provide the first homogeneous high-resolution ($1.3'' = 0.05$ pc) observations aiming at resolving density and temperature structures. The CS clouds have sizes of $0.05-0.2$ pc with a broad range of velocity dispersion ($\sigma_{\rm FWHM}=5-40$ km s$^{-1}$). The CS clouds are a mixture of warm ($T_{\rm k}\ge 50-500$ K, n${\rm H_2}$=$10^{3-5}$ cm$^{-3}$) and cold gas ($T{\rm k}\le 50$ K, n${\rm H_2}$=$10^{6-8}$ cm$^{-3}$). A stability analysis based on the unmagnetized virial theorem including tidal force shows that $84^{+16}{-37}$ % of the total gas mass is tidally stable, which accounts for the majority of gas mass. Turbulence dominates the internal energy and thereby sets the threshold densities $10-100$ times higher than the tidal limit at distance $\ge 1.5$ pc to Sgr A*, and therefore, inhibits the clouds from collapsing to form stars near the SMBH. However, within the central $1.5$ pc, the tidal force overrides turbulence and the threshold densities for a gravitational collapse quickly grow to $\ge 10^{8}$ cm$^{-3}$.

• The paper demonstrates that ALMA’s high-resolution CS mapping reveals that approximately 84% of the gas mass in the circumnuclear disk is stable against tidal disruption.
• Using multiple CS transitions, the study distinguishes between cold, high-density clouds and warm, medium-density gas, each with unique thermal and kinematic properties.
• The paper highlights that despite conditions favoring star formation, external pressures and potential magnetic influences may inhibit collapse within the Galactic Center.

Analysis of Circumnuclear Disk and Galactic Center Dynamics Using ALMA Data

The discovery and analysis of circumnuclear disks (CNDs) are pivotal in understanding the dynamic behavior and stability of molecular material in the vicinity of supermassive black holes (SMBHs). The paper "The Circumnuclear Disk Revealed by ALMA: Dense Clouds and Tides in the Galactic Center" by Hsieh et al. presents an extensive study utilizing the Atacama Large Millimeter/submillimeter Array (ALMA) to probe the CND and streamer structures in the Galactic Center. ALMA’s high-resolution capabilities provide unprecedented insights into the physical conditions and stability phenomena of gas clouds around the SMBH, Sagittarius A* (Sgr A*).

Observational Methodology and Data

Hsieh et al. extensively employed ALMA to produce high-resolution CS line maps at multiple rotational transitions ($J_{\rm u}=7,5,4,3,2$). These maps cover the circumnuclear disk and its surrounding structures, unraveling the density and temperature distributions in the central few parsecs of the galaxy. The observations boast resolutions of 1.3 arcseconds (equivalent to 0.05 parsecs), allowing detailed analysis of cloud structures near Sgr A*. The study not only segments the complex molecular cloud formations but also dissects their stability against external gravitational and turbulent forces.

Key Findings on Stability and Dynamics

Structure and Temperature

The research identifies that the manifesting CS clouds span sizes between 0.05 and 0.2 pc, exhibiting wide-ranging velocity dispersions (5-40 km/s). Interpreting the data, the authors discern a dual nature in the gas: cold gas with higher densities (n$_{\rm H_2}$ ranging from $10^6$ to $10^8$ cm$^{-3}$ and temperatures $T_{\rm k}\le 50$ K) and warm, medium-density gas (n$_{\rm H_2}$ between $10^3$ and $10^5$ cm$^{-3}$ with temperatures $T_{\rm k} \ge 50-500$ K). These conditions emphasize the heterogeneous environment of the Galactic Center.

Implications on Tidal Stability

A critical aspect under study is the tidal stability of gas within the gravitational influence of Sgr A*. Using a virial theorem analysis inclusive of tidal forces, the study concludes that approximately 84% of the total gas mass is stable against tidal disruption. However, turbulence exerts a significant influence on internal energies. At distances exceeding 1.5 pc from Sgr A*, turbulent energies surpass tidal forces, thereby requiring threshold densities 10 to 100 times above traditional tidal limits for gas stability. Within 1.5 pc of Sgr A*, however, these thresholds ascend rapidly due to dominant tidal forces, curtailing the potential for star formation.

Star Formation and the Role of External Pressure

Remarkably, while a noticeable fraction of observed clumps demonstrates the capacity for star formation based on critical density benchmarks, there lacks clear evidence of active star formation within the CND. This anomaly suggests potential roles for unaccounted stabilizing forces, possibly magnetic fields, inhibiting collapse despite favorable conditions. Furthermore, the necessity of external pressures in stabilizing observed structures implicates complex interactions within the central galactic regions, where pressures exceed those typical in galactic disks by orders of magnitude.

Concluding Remarks

The study by Hsieh et al. significantly advances the understanding of dynamic processes governing the CND around Sgr A*. Through ALMA’s capabilities, the researchers elucidate the competing influences of tidal forces and internal pressures (thermal and turbulent), shedding light on the delicate equilibrium maintaining these cosmic structures. The findings carry substantial implications for SMBH accretion processes and stellar genesis in similar environments across the universe. Future approaches, possibly involving magnetohydrodynamic considerations, will further complete the understanding of these complex cosmic interactions.