Active galactic nuclei - Internal dynamics and formation of emission clouds

Abstract

We show that cool UV accretion disks, if present in active galactic nuclei,can be sources of line-driven winds. Subcritical disks are shown to be capable of producing winds optically thin in the continuum, which are accelerated by radiation pressure in lines (i.e., by absorption and scattering of disk-continuum photons in the Doppler-shifted optically thick UV resonance lines). Results of a numerical simulation to the outflow from a disk around a supermassive black hole are presented and discussed. The necessary condition for initiating the wind is that the local radiative flux must be peaked in the energy range of existing lines. The base of the wind must therefore be shielded from the central region, which is assumed to emit power-law-distributed radiation. The disk atmosphere can provide such shielding. The winds are characterized by steep velocity gradients and high asymptotic velocities, approx.10/sup 9/ cm s/sup -1/, as observed in broad emission-line regions of QSOs and Seyfert galaxies, and are thermally unstable, as they heat up. This results in a two-phase equilibrium: cold clouds embedded in a hot, Compton-heated background. The hot wind provides the pressure confinement for the dense and massive clouds. The relevant time scales for the cloud formation and theirmore » future evolution are estimated, and the implications for the dynamics of the broad emission-line regions are discussed.« less