Overlapping Inflow Events as Catalysts for Supermassive Black Hole
  Growth
  One of the greatest issues in modelling black hole fuelling is our lack of
understanding of the processes by which gas loses angular momentum and falls
from galactic scales down to the nuclear region where an accretion disc forms,
subsequently guiding the inflow of gas down to the black hole horizon. It is
feared that gas at larger scales might still retain enough angular momentum and
settle into a larger scale disc with very low or no inflow to form or replenish
the inner accretion disc (on ~0.01 pc scales). In this paper we report on
hydrodynamical simulations of rotating infalling gas shells impacting at
different angles onto a pre-existing, primitive large scale (~10 pc) disc
around a super-massive black hole. The aim is to explore how the interaction
between the shell and the disc redistributes the angular momentum on scales
close to the black hole's sphere of influence. Angular momentum redistribution
via hydrodynamical shocks leads to inflows of gas across the inner boundary,
enhancing the inflow rate by more than 2-3 orders of magnitude. In all cases,
the gas inflow rate across the inner parsec is higher than in the absence of
the interaction, and the orientation of the angular momentum of the flow in the
region changes with time due to gas mixing. Warped discs or nested misaligned
rings form depending on the angular momentum content of the infalling shell
relative to the disc. In the cases in which the shell falls in near
counter-rotation, part of the resulting flows settle into an inner dense disc
which becomes more susceptible to mass transfer.
