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