The impact of angular momentum on black hole accretion rates in
  simulations of galaxy formation
  Feedback from energy liberated by gas accretion onto black holes (BHs) is an
attractive mechanism to explain the exponential cut-off at the massive end of
the galaxy stellar mass function (SMF). Semi-analytic models of galaxy
formation in which this form of feedback is assumed to suppress cooling in
haloes where the gas cooling time is large compared to the dynamical time do
indeed achieve a good match to the observed SMF. Furthermore, hydrodynamic
simulations of individual halos in which gas is assumed to accrete onto the
central BH at the Bondi rate have shown that a self-regulating regime is
established in which the BH grows just enough to liberate an amount of energy
comparable to the thermal energy of the halo. However, this process is
efficient at suppressing the growth not only of massive galaxies but also of
galaxies like the Milky Way, leading to disagreement with the observed SMF. The
Bondi accretion rate, however, is inappropriate when the accreting material has
angular momentum. We present an improved accretion model that takes into
account the circularisation and subsequent viscous transport of infalling
material and include it as a "subgrid" model in hydrodynamic simulations of the
evolution of halos with a wide range of masses. The resulting accretion rates
are generally low in low mass ($\lsim 10^{11.5} \msun$) halos, but show
outbursts of Eddington-limited accretion during galaxy mergers. During
outbursts these objects strongly resemble quasars. In higher mass haloes, gas
accretion occurs continuously, typically at $~10$ % of the Eddington rate,
which is conducive to the formation of radio jets. The resulting dependence of
the accretion behaviour on halo mass induces a break in the relation between
galaxy stellar mass and halo mass in these simulations that matches
observations.
