Resolving flows around black holes: the impact of gas angular momentum
  Cosmological simulations almost invariably estimate the accretion of gas on
to supermassive black holes using a Bondi-Hoyle-like prescription. Doing so
ignores the effects of the angular momentum of the gas, which may prevent or
significantly delay accreting material falling directly on to the black hole.
We outline a black hole accretion rate prescription using a modified
Bondi-Hoyle formulation that takes into account the angular momentum of the
surrounding gas. Meaningful implementation of this modified Bondi-Hoyle
formulation is only possible when the inner vorticity distribution is well
resolved, which we achieve through the use of a super-Lagrangian refinement
technique around black holes within our simulations. We then investigate the
effects on black hole growth by performing simulations of isolated as well as
merging disc galaxies using the moving-mesh code AREPO. We find that the gas
angular momentum barrier can play an important role in limiting the growth of
black holes, leading also to a several Gyr delay between the starburst and the
quasar phase in major merger remnants. We stress, however, that the magnitude
of this effect is highly sensitive to the thermodynamical state of the
accreting gas and to the nature of the black hole feedback present.
