Chaotic cold accretion onto black holes. (arXiv:1301.3130v1 [astro-ph.CO])

Chaotic cold accretion onto black holes. (arXiv:1301.3130v1 [astro-ph.CO]):
Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the
course of 40 Myr, we systematically relax the classic Bondi assumptions in a
typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is
cooling, while heated and stirred on large scales, the accretion rate is
boosted up to two orders of magnitude compared with the Bondi prediction. The
cause is the nonlinear growth of thermal instabilities, leading to the
condensation of cold clouds and filaments when t_cool/t_ff < 10. Subsonic
turbulence of just over 100 km/s (M > 0.2) induces the formation of thermal
instabilities, even in the absence of heating, while in the transonic regime
turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating
restores global thermodynamic balance, the formation of the multiphase medium
is violent, and the mode of accretion is fully cold and chaotic. The recurrent
collisions, shearing and tidal motions between clouds, filaments and the
central torus cause a significant reduction of angular momentum, boosting
accretion. On sub-pc scales the clouds are channelled to the very centre via a
funnel. A good approximation to the accretion rate is the cooling rate, which
can be used as subgrid model, physically reproducing the boost factor of 100
required by cosmological simulations, while accounting for fluctuations.
Chaotic cold accretion may be common in many systems, such as hot galactic
halos, groups, and clusters, generating high-velocity clouds and strong
variations of the AGN luminosity, jet orientation, and spin. In this mode, the
black hole can quickly react to the state of the entire host galaxy, leading to
efficient self-regulated feedback and the symbiotic Magorrian relation. During
phases of overheating, the hot mode becomes the single channel of accretion
(with a different cuspy temperature profile), though strongly suppressed by
turbulence.