Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster

Abstract: We investigate the formation of circumnuclear gas structures from the tidal disruption of molecular clouds in galactic nuclei, by means of smoothed particle hydrodynamics simulations. We model galactic nuclei as composed of a supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider different mass ratios between the two components. We find that the relative masses of the SMBH and the NSC have a deep impact on the morphology of the circumnuclear gas. Extended disks form only inside the sphere of influence of the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere of influence, where the gravity is dominated by the NSC. This result is in agreement with the properties of the Milky Way's circumnuclear ring, which orbits outside the SMBH sphere of influence. Our results indicate that compact circumnuclear rings can naturally form outside the SMBH sphere of influence.

• The paper shows that the relative mass contributions of supermassive black holes (SMBH) and nuclear star clusters (NSC) significantly affect the morphology of circumnuclear gas structures.
• Extended disks form exclusively within the SMBH's sphere of influence, while compact rings form outside this region where the NSC's gravity is dominant, explaining features like the Milky Way's circumnuclear ring.
• The inner edge of a circumnuclear ring could serve as an upper bound approximation for the SMBH's sphere of influence, offering a potential method for SMBH mass estimation where direct observations are difficult.

An Analysis of Circumnuclear Gas Morphology in Galactic Nuclei

The paper authored by Alessandro A. Trani, Michela Mapelli, and Alessandro Ballone explores the formation of circumnuclear gas structures in galactic nuclei through an elaborate study of the interplay between supermassive black holes (SMBH) and nuclear star clusters (NSC). Their work challenges the notion that the SMBH is the sole determinant of gas morphology around galactic nuclei by presenting evidence that the relative mass contributions of SMBH and NSC significantly affect the resultant gas structures.

Key Findings and Methodology

The authors employ smoothed particle hydrodynamics (SPH) simulations to model galactic nuclei as composite entities of an SMBH and an NSC, analyzing cases with varying mass ratios. The focus of the study lies in observing the formation of gaseous disks and rings through the tidal disruption of molecular clouds. Notably, they discover that the mass ratio between the SMBH and NSC deepens the impact on circumnuclear gas morphology in significant ways:

1. Disks and Rings Formation:
   • Extended disks are found to form exclusively within the SMBH's sphere of influence, indicating that the gravitational dominance of the SMBH within this radius leads to a near-Keplerian dynamics of the gas.
   • Compact rings, on the other hand, form outside the influence of the SMBH where the NSC's gravitational pull is predominant. This finding is in line with the observed properties of the Milky Way's circumnuclear ring, which orbits externally to the SMBH's sphere of influence.
2. Implications for Galactic Center Observations:
   • The study provides a plausible explanation for the observed morphology of gas in the Galactic Center, including structures like the Milky Way’s circumnuclear ring. The results suggest that the ring's location external to the sphere of influence is expected under the gravitational dynamics of a star cluster-dominated zone.
3. Significance of the SMBH Influence Radius:
   • The authors posit that the inner edge of a circumnuclear ring could serve as an upper bound approximation for the SMBH's sphere of influence. This retains potential utility for SMBH mass measurements when direct observations of the black hole aren't feasible.

The simulations explore a diverse range of setups involving different SMBH-to-NSC mass ratios, revealing dynamic outcomes contingent on these configurations. An essential inference from the model scenarios is that circumnuclear rings do not easily form in configurations lacking a significant NSC, which reaffirms the necessity of considering both SMBH and NSC components for accurate predictions of the circumnuclear gas morphology.

Broader Implications and Future Directions

This paper provides foundational insights that can impact the methodology employed in dynamical mass measurements of SMBHs. The ability to use circumnuclear gas configurations as indicators of SMBH influence offers a proximal approach to estimating SMBH masses, especially in environments where traditional methods may be hampered by observational constraints.

Looking forward, future research could benefit from integrating more complex models that factor in chemical compositions and star formation feedback mechanisms, as well as expanding the studies to various types of galaxy nuclei beyond those similar to the Milky Way. Additionally, exploring further implications of NSC dynamics and their role in galaxy evolution remains a promising avenue for both theoretical exploration and observational validation.

In closing, this study robustly bridges the understanding of circumnuclear gas morphologies with properties of underlying galactic dynamical structures, with the potential to refine applications in galaxy and black hole astrophysics. It urges a thoughtful reconsideration of the traditional isolation of SMBHs in understanding galactic centers and emphasizes a more inclusive consideration of nuclear star clusters in these environments.