Feeding of active galactic nuclei by dynamical perturbations
  There possibly was an AGN episode in the Galactic Centre about 6 Myr ago,
powerful enough to produce the Fermi bubbles. We present numerical simulations
of a possible scenario giving rise to an activity episode: a collision between
a central gas ring surrounding the supermassive black hole (SMBH) and an
infalling molecular cloud. We investigate different initial collision angles
between the cloud and the ring. We follow the hydrodynamical evolution of the
system following the collision using Gadget-3 hybrid N-body/SPH code and
calculate the feeding rate of the SMBH accretion disc. This rate is then used
as an input for a 1D thin $\alpha$-disc model in order to calculate the AGN
luminosity. By varying the disc feeding radii we determine the limiting values
for possible AGN accretion disc luminosity. Small angle collisions do not
result in significant mass transport to the centre of the system, while models
with highest collision angles transport close to $40\%$ of the initial matter
to the accretion disc. Even with ring and cloud masses equal to $10^4 \,
M_\odot$, which is the lower limit of present-day mass of the Circumnuclear
ring in the Galactic Centre, the energy released over an interval of 1.5 Myr
can produce $\sim 10\%$ of that required to inflate the Fermi bubbles. If the
gas ring in the Galactic Centre 6 Myr ago had a mass of at least $10^5 \,
M_\odot$, our proposed scenario can explain the formation of the Fermi bubbles.
We estimate that such high-impact collisions might occur once every $\sim 10^8$
yr in our Galaxy.
