Isotropic Heating of Galaxy Cluster Cores via Rapidly Reorienting AGN Jets. (arXiv:1209.5748v1 [astro-ph.CO])

Isotropic Heating of Galaxy Cluster Cores via Rapidly Reorienting AGN Jets. (arXiv:1209.5748v1 [astro-ph.CO]):
AGN jets carry more than sufficient energy to stave off catastrophic cooling
of the intracluster medium (ICM) in the cores of cool-core clusters. However,
in order to prevent catastrophic cooling, the ICM must be heated in a
near-isotropic fashion and narrow bipolar jets are inefficient at heating the
gas in the transverse direction to the jets. We argue that due to existent
conditions in cluster cores, the SMBHs will, in addition to accreting gas via
radiatively inefficient flows, experience short stochastic episodes of enhanced
accretion via thin discs. In general, the orientation of these accretion discs
will be misaligned with the spin axis of the black holes and the ensuing
torques will cause the black hole's spin axis (and therefore, the jet axis) to
slew and rapidly change direction. This model not only explains recent
observations showing successive generations of jet-lobes-bubbles in individual
cool-core clusters that are offset from each other in the angular direction
with respect to the cluster center, but also shows that AGN jets {\it can} heat
the cluster core nearly isotropically on the gas cooling timescale. One
implication of our proposed model is that since SMBHs that host thin accretion
discs will manifest as quasars, we predicts that roughly 1--2 rich clusters
within $z<0.5$ should have quasars at their centers. Also, recurrent accretion
via misaligned accretion discs implies that as a population, the SMBHs at the
centers of cool-core clusters should be spinning slowly. Our model, in fact,
requires SMBHs to be spinning slowly. Torques from misaligned discs are
ineffective at tilting rapidly spinning black holes by more a few degrees
whereas slowly spinning SMBHs can, under optimal conditions, slew by as much as
$\sim 30^\circ$ during any one accretion event.