Accretion Disc Particle Accretion in Major Merger Simulations
  A recent approach to simulating localized feedback from active galactic
nuclei by Power et al. (2011) uses an accretion disc particle to represent both
the black hole and its accretion disc. We have extrapolated and adapted this
approach to simulations of Milky Way-sized galaxy mergers containing black
holes and explored the impact of the various parameters in this model as well
as its resolution dependence. The two key parameters in the model are an
effective accretion radius, which determines the radius within which gas
particles are added to the accretion disc, and a viscous time-scale which
determines how long it takes for material in the accretion disc to accrete on
to the black hole itself. We find that there is a limited range of permitted
accretion radii and viscous time-scales, with unphysical results produced
outside this range. For permitted model parameters, the nuclear regions of
simulations with the same resolution follow similar evolutionary paths,
producing final black hole masses that are consistent within a factor of two.
When comparing the resolution dependence of the model, there is a trend towards
higher resolution producing slightly lower mass black holes, but values for the
two resolutions studied again agree within a factor of two. We also compare
these results to two other AGN feedback algorithms found in the literature.
While the evolution of the systems vary, most notably the intermediate total
black hole mass, the final black hole masses differ by less than a factor of
five amongst all of our models, and the remnants exhibit similar structural
parameters. The implication of this accretion model is that, unlike most
accretion algorithms, a decoupling of the accretion rate on to the black hole
and the local gas properties is permitted and obtained; this allows for black
hole growth even after feedback has prevented additional accretion events on to
the disc.
