Simulations of supermassive black hole growth in high-redshift disk
  galaxies
  Observations suggest that a large fraction of black hole growth occurs in
normal star-forming disk galaxies. Here we describe simulations of black hole
accretion in isolated disk galaxies with sufficient resolution (~5 pc) to track
the formation of giant molecular clouds that feed the black hole. Black holes
in z=2 gas-rich disks (fgas=50%) occasionally undergo ~10 Myr episodes of
Eddington-limited accretion driven by stochastic collisions with massive, dense
clouds. We predict that these gas-rich disks host weak AGNs 1/4 of the time,
and moderate/strong AGNs 10% of the time. Averaged over 100 Myr timescales and
the full distribution of accretion rates, the black holes grow at a few per
cent of the Eddington limit -- sufficient to match observations and keep the
galaxies on the MBH-Mbulge relation. This suggests that dense cloud accretion
in isolated z=2 disks could dominate cosmic black hole growth. In z=0 disks
with fgas=10%, Eddington-limited growth is extremely rare because typical gas
clouds are smaller and more susceptible to disruption by AGN feedback. This
results in an average black hole growth rate in high-fgas galaxies that is up
to 1000 times higher than that in low-fgas galaxies. In all our simulations,
accretion shows variability by factors of 10^4 on a variety of time scales,
with variability at 1 Myr scales driven by the structure of the interstellar
medium.
