The case for super-critical accretion onto massive black holes at high
  redshift
  Short-lived intermittent phases of super-critical (super-Eddington) growth,
coupled with star formation via positive feedback, may account for early growth
of massive black holes (MBH) and coevolution with their host spheroids. We
estimate the possible growth rates and duty cycles of these episodes, both
assuming slim accretion disk solutions, and adopting the results of recent
numerical simulations. The angular momentum of gas joining the accretion disk
determines the length of the accretion episodes, and the final mass a MBH can
reach. The latter can be related to the gas velocity dispersion, and in
galaxies with low-angular momentum gas the MBH can get to a higher mass. When
the host galaxy is able to sustain inflow rates at 1-100 msunyr, replenishing
and circulation lead to a sequence of short (~1e4-1e7 years), heavily obscured
accretion episodes that increase the growth rates, with respect to an
Eddington-limited case, by several orders of magnitude. Our model predicts that
the ratio of MBH accretion rate to star formation rate is 1e2 or higher,
leading, at early epochs, to a ratio of MBH to stellar mass higher than the
"canonical" value of ~1e-3, in agreement with current observations. Our model
makes specific predictions that long-lived super-critical accretion occurs only
in galaxies with copious low-angular momentum gas, and in this case the MBH is
more massive at fixed velocity dispersion.
