Accretion Disk Winds from Active Galactic Nuclei

Abstract

We present a dynamical model in which the broad absorption lines seen in some quasar spectra form in a smooth nonspherical wind from an accretion disk. On the inner edge of this wind, at r ≍ 1016 cm for a 108 Msun black hole, entrained dense gas blocks soft X-rays but transmits ultraviolet photons; this allows radiation pressure on resonance lines to accelerate the outflow to ∼0.1c. The wind is driven up out of the disk by a combination of radiation pressure and gas pressure. The wind is nearly in the plane of the disk, with an opening angle of ∼5°. If the quasar is viewed through the wind, broad blueshifted absorption lines are seen, particularly in the resonance transitions of C IV, O VI, Ne VIII, and Lyman-α. In objects showing such blue-shifted absorption, strong, soft X-ray absorption will also be seen. The wind produces emission lines at most viewing angles, which contributes significantly to the formation of the broad emission lines that characterize active galactic nuclei. We describe self-consistent dynamical and photoionization calculations of the line radiation force on the gas. These demonstrate that the soft X-ray blocking gas modifies the spectral energy distribution of the central source so that the line driving is effective for extremely high ionization parameters. This modified spectrum also accounts for the discrepancy with previous photoionization calculations, which indicated that both the emitting and absorbing gas had a very small filling factor.