Abstract
ACTIVE galactic nuclei (AGNs) are usually thought to be powered by accretion onto a supermassive black hole (SBH)1,2. Their luminosities, which may exceed 1046 erg s-1, require mass accretion rates of ≳=lA M⊙ yr-1 for reasonable mass-to-energy conversion efficiencies. Although this quantity of fuel could be supplied by the interstellar medium of the host galaxy, it is not obvious how it could be transported from typical galactic radii, ∼10 kpc, down to the scale of the SBH, ≲10 pc. We propose here a mechanism, applicable to AGNs and nuclear starburst galaxies, which brings in gas from large to small scales by successive dynamical instabilities. On the large scale, a stellar bar sweeps the interstellar medium into a gaseous disk of a few hundred parsecs in radius. Under certain conditions, this disk can become unstable again, allowing material to flow inwards until turbulent viscous processes control angular-momentum transport. This flow pattern may feed viscosity-driven accretion flows around a SBH, or lead to the formation of a SBH if none was present initially.
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