Triggering and Delivery Algorithms for AGN Feedback
  We compare several common sub-grid implementations of AGN feedback, focusing
on the effects of different triggering mechanisms and the differences between
thermal and kinetic feedback. Our main result is that pure thermal feedback
that is centrally injected behaves differently from feedback with even a small
kinetic component. Specifically, pure thermal feedback results in excessive
condensation and smothering of the AGN by cold gas because the feedback energy
does not propagate to large enough radii. We do not see large differences
between implementations of different triggering mechanisms, as long as the
spatial resolution is sufficiently high, probably because all of the
implementations tested here trigger strong AGN feedback under similar
conditions. In order to assess the role of resolution, we vary the size of the
"accretion zone" in which properties are measured to determine the AGN
accretion rate and resulting feedback power. We find that a larger accretion
zone results in steadier jets but can also allow too much cold-gas condensation
in simulations with a Bondi-like triggering algorithm. We also vary the opening
angle of jet precession and find that a larger precession angle causes more of
the jet energy to thermalize closer to the AGN, thereby producing results
similar to pure thermal feedback. Our simulations confirm that AGN can regulate
the thermal state of cool-core galaxy clusters and maintain the core in a state
that is marginally susceptable to thermal instability and precipitation.
