Overlapping inflows as catalysts of AGN activity - II: Relative
  importance of turbulence and inflow-disc interaction
  The main challenge for understanding the fuelling of supermassive black holes
in active galactic nuclei is not to account for the source of fuel, but rather
to explain its delivery from the boundaries of the black hole sphere of
influence (10-100 pc) down to sub-parsec scales. In this work, we report on a
series of numerical experiments aimed at exploring in further depth our model
of "overlapping inflow events" as catalysts for rapid accretion, seeding a
turbulent field in the infalling gas. We initially set a gaseous shell in
non-equilibrium rotation around a supermassive black hole. After infall, the
shell stalls in a disc-like structure. A second shell is then set in either
co-rotation or counter-rotation with respect to the first and is let to impinge
on the previously-formed disc. We find that combined turbulence and overlap
significantly enhance accretion in counter-rotating inflows, while turbulence
dominates for co-rotating inflows. The leftovers of overlapping inflows are
warped nuclear discs, whose morphology depends on the relative orientation and
angular momentum of the disc and the shell. Overlapping inflows leave
observational signatures in the gas rotation curves.
