Arif Babul, Prateek Sharma, Christopher S. Reynolds
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.
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http://arxiv.org/abs/1209.5748
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