Torque-Limited Growth of Massive Black Holes in Galaxies Across Cosmic
  Time
  We combine cosmological hydrodynamic simulations with analytic models to
evaluate the role of galaxy-scale gravitational torques on the evolution of
massive black holes at the centers of star-forming galaxies. We confirm and
extend our earlier results to show that torque-limited growth yields black
holes and host galaxies evolving on average along the Mbh-Mbulge relation from
early times down to z = 0 and that convergence onto the scaling relation occurs
independent of the initial conditions and with no need for mass averaging
through mergers or additional self-regulation processes. Smooth accretion
dominates the long-term evolution, with black hole mergers with mass ratios
>1:5 representing typically a small fraction of the total growth. Winds from
the accretion disk are required to eject significant mass to suppress black
hole growth, but there is no need for coupling this wind to galactic-scale gas
to regulate black holes in a non-linear feedback loop. Torque-limited growth
yields a close-to-linear relation for the star formation rate and the black
hole accretion rate averaged over galaxy evolution time scales. However, the
SFR-AGN connection has significant scatter owing to strong variability of black
hole accretion at all resolved time scales. Eddington ratios can be described
by a broad lognormal distribution with median value evolving roughly as (1 +
z)^1.9, suggesting a main sequence for black hole growth similar to the cosmic
evolution of specific SFRs. Our results offer an attractive scenario consistent
with available observations in which cosmological gas infall and transport of
angular momentum in the galaxy by gravitational instabilities regulate the
long-term co-evolution of black holes and star-forming galaxies.
