Black hole feeding and feedback: the physics inside the "subgrid"
  Black holes (BHs) are believed to be a key ingredient of galaxy formation.
However, the galaxy-BH interplay is challenging to study due to the large
dynamical range and complex physics involved. As a consequence, hydrodynamical
cosmological simulations normally adopt sub-grid models to track the unresolved
physical processes, in particular BH accretion; usually the spatial scale where
the BH dominates the hydrodynamical processes (the Bondi radius) is unresolved,
and an approximate Bondi-Hoyle accretion rate is used to estimate the growth of
the BH. By comparing hydrodynamical simulations at different resolutions (300,
30, 3 pc) using a Bondi-Hoyle approximation to sub-parsec runs with
non-parameterized accretion, our aim is to probe how well an approximated Bondi
accretion is able to capture the BH accretion physics and the subsequent
feedback on the galaxy. We analyse an isolated galaxy simulation that includes
cooling, star formation, Type Ia and Type II supernovae, BH accretion and AGN
feedback (radiation pressure, Compton heating/cooling) where mass, momentum,
and energy are deposited in the interstellar medium through conical winds. We
find that on average the approximated Bondi formalism can lead to both over-
and under-estimations of the BH growth, depending on resolution and on how the
variables entering into the Bondi-Hoyle formalism are calculated.
