48,444 research outputs found
Chaotic cold accretion onto black holes
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 and tidal forces between clouds, filaments and the central clumpy
torus promote angular momentum cancellation, hence 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 and jet orientation. In this mode, the black hole can quickly react
to the state of the entire host galaxy, leading to efficient self-regulated AGN
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.Comment: Accepted by MNRAS: added comments and references. Your feedback is
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AGN Feedback and Bimodality in Cluster Core Entropy
We investigate a series of steady-state models of galaxy clusters, in which
the hot intracluster gas is efficiently heated by active galactic nucleus (AGN)
feedback and thermal conduction, and in which the mass accretion rates are
highly reduced compared to those predicted by the standard cooling flow models.
We perform a global Lagrangian stability analysis. We show for the first time
that the global radial instability in cool core clusters can be suppressed by
the AGN feedback mechanism, provided that the feedback efficiency exceeds a
critical lower limit. Furthermore, our analysis naturally shows that the
clusters can exist in two distinct forms. Globally stable clusters are expected
to have either: 1) cool cores stabilized by both AGN feedback and conduction,
or 2) non-cool cores stabilized primarily by conduction. Intermediate central
temperatures typically lead to globally unstable solutions. This bimodality is
consistent with the recently observed anticorrelation between the flatness of
the temperature profiles and the AGN activity (Dunn & Fabian 2008) and the
observation by Rafferty et al. (2008) that the shorter central cooling times
tend to correspond to significantly younger AGN X-ray cavities.Comment: 4 pages, to appear in the proceedings of "The Monster's Fiery Breath:
Feedback in Galaxies, Groups, and Clusters", Eds. Sebastian Heinz, Eric
Wilcots (AIP conference series
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