Stratified disc wind models for the AGN broad-line region: ultraviolet, optical and X-ray properties

The origin, geometry and kinematics of the broad line region (BLR) gasin quasars and active galactic nuclei (AGN) are uncertain. Wedemonstrate that clumpy biconical disc winds illuminated by an AGNcontinuum can produce BLR-like spectra. We first use a simple toy modelto illustrate that disc winds make quite good BLR candidates, becausethey are self-shielded flows and can cover a large portion of theionizing flux-density ($\phi_H$-$n_H$) plane. We then conduct MonteCarlo radiative transfer and photoionization calculations, which fullyaccount for self-shielding and multiple scattering in a non-sphericalgeometry. The emergent model spectra show broad emission lines withequivalent widths and line ratios comparable to those observed in AGN,provided that the wind has a volume filling factor of $f_V\lesssim0.1$.Similar emission line spectra are produced for a variety of windgeometries (polar or equatorial) and for launch radii that differ by anorder of magnitude. The line emission arises almost exclusively fromplasma travelling below the escape velocity, implying that `failedwinds' are important BLR candidates. The behaviour of a line-emittingwind (and possibly any `smooth flow' BLR model) is similar to that ofthe locally optimally-emitting cloud (LOC) model originally proposed byBaldwin et al (1995), except that the gradients in ionization state andtemperature are large-scale and continuous, rather than within orbetween distinct clouds. Our models also produce UV absorption lines andX-ray absorption features, and the stratified ionization structure canpartially explain the different classes of broad absorption line quasars.