Environmental Dependence of Self-Regulating Black-hole Feedback in
  Massive Galaxies
  In the universe's most massive galaxies, kinetic feedback from a central
supermassive black hole appears to limit star formation. Abundant
circumstantial evidence suggests that accumulation of cold gas near the central
black hole strongly boosts the feedback output, keeping the ambient medium in a
state marginally unstable to condensation and formation of cold gas clouds.
However, the ability of that mechanism to self-regulate may depend on numerous
environmental factors, including the depth of the potential well and the
pressure of the surrounding circumgalactic medium (CGM). Here we present a
suite of numerical simulations that explores the dependence of cold-fuelled
bipolar kinetic feedback on those environmental factors. Halo mass in this
simulation suite ranges from $2 \times 10^{12} \, M_\odot$ to $8 \times 10^{14}
\, M_\odot$. We include the spatially extended mass and energy input from the
massive galaxy's old stellar population, which is capable of sweeping gas out
of the galaxy and away from the central black hole if the confining CGM
pressure is sufficiently low. Our simulations show that this feedback mechanism
is tightly self-regulating in a massive galaxy with a deep central potential
and low CGM pressure, permitting only small amounts of multiphase gas to
accumulate and allowing almost no star formation. In a massive galaxy of
similar mass but a shallower central potential and greater CGM pressure the
same feedback mechanism is more episodic, producing extended multiphase gas and
occasionally allowing small rates of star formation ($\sim 0.1 \, M_\odot \,
{\rm yr}^{-1}$). At the low-mass end of the explored range the mechanism
becomes implausibly explosive, perhaps because the ambient gas initially has no
angular momentum, which would have reduced the amount of condensed gas capable
of fueling feedback.
