Modelling the formation of the circumnuclear ring in the Galactic centre
  Several thousand solar masses of molecular, atomic and ionized gas lie in the
innermost ~10 pc of our Galaxy. The most relevant structure of molecular gas is
the circumnuclear ring (CNR), a dense and clumpy ring surrounding the
supermassive black hole (SMBH), with a radius of ~2 pc. We propose that the CNR
formed through the tidal disruption of a molecular cloud, and we investigate
this scenario by means of N-body smoothed-particle hydrodynamics simulations.
We ran a grid of simulations with different cloud mass (4X10^4, 1.3X10^5 solar
masses), different initial orbital velocity (v_in=0.2-0.5 v_esc, where v_esc is
the escape velocity from the SMBH), and different impact parameter (b=8, 26
pc). The disruption of the molecular cloud leads to the formation of very dense
and clumpy gas rings, containing most of the initial cloud mass. If the initial
orbital velocity of the cloud is sufficiently low (v_in<0.4 v_esc, for b=26 pc)
or the impact parameter is sufficiently small (b<10 pc, for v_in>0.5 v_esc), at
least two rings form around the SMBH: an inner ring (with radius ~0.4 pc) and
an outer ring (with radius ~2-4 pc). The inner ring forms from low-angular
momentum material that engulfs the SMBH during the first periapsis passage,
while the outer ring forms later, during the subsequent periapsis passages of
the disrupted cloud. The inner and outer rings are misaligned by ~24 degrees,
because they form from different gas streamers, which are affected by the SMBH
gravitational focusing in different ways. The outer ring matches several
properties (mass, rotation velocity, temperature, clumpiness) of the CNR in our
Galactic centre. We speculate that the inner ring might account for the neutral
gas observed in the central cavity.
