Black Hole-Galaxy Correlations without Self-Regulation
  Recent models of black hole growth in a cosmological context have forwarded a
paradigm in which the growth is self-regulated by feedback from the black hole
itself. Here we use cosmological zoom simulations of galaxy formation down to z
= 2 to show that such strong self-regulation is required in the popular
spherical Bondi accretion model, but that a plausible alternative model in
which black hole growth is limited by galaxy-scale torques does not require
self-regulation. Instead, this torque-limited accretion model yields black
holes and galaxies evolving on average along the observed scaling relations by
relying only on a fixed, 5% mass retention rate onto the black hole from the
radius at which the accretion flow is fed. Feedback from the black hole may
(and likely does) occur, but does not need to couple to galaxy-scale gas in
order to regulate black hole growth. We show that this result is insensitive to
variations in the initial black hole mass, stellar feedback, or other
implementation details. The torque-limited model allows for high accretion
rates at very early epochs (unlike the Bondi case), which if viable can help
explain the rapid early growth of black holes, while by z = 2 it yields
Eddington factors of 1%-10%. This model also yields a less direct
correspondence between major merger events and rapid phases of black hole
growth. Instead, growth is more closely tied to cosmological disk feeding,
which may help explain observational studies showing that, at least at z > 1,
active galaxies do not preferentially show merger signatures.
