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) gas in
quasars and active galactic nuclei (AGN) are uncertain. We demonstrate that
clumpy biconical disc winds illuminated by an AGN continuum can produce
BLR-like spectra. We first use a simple toy model to illustrate that disc winds
make quite good BLR candidates, because they are self-shielded flows and can
cover a large portion of the ionizing flux-density ($\phi_H$-$n_H$) plane. We
then conduct Monte Carlo radiative transfer and photoionization calculations,
which fully account for self-shielding and multiple scattering in a
non-spherical geometry. The emergent model spectra show broad emission lines
with equivalent 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 wind geometries (polar or
equatorial) and for launch radii that differ by an order of magnitude. The line
emission arises almost exclusively from plasma travelling below the escape
velocity, implying that `failed winds' are important BLR candidates. The
behaviour of a line-emitting wind (and possibly any `smooth flow' BLR model) is
similar to that of the locally optimally-emitting cloud (LOC) model originally
proposed by Baldwin et al (1995), except that the gradients in ionization state
and temperature are large-scale and continuous, rather than within or between
distinct clouds. Our models also produce UV absorption lines and X-ray
absorption features, and the stratified ionization structure can partially
explain the different classes of broad absorption line quasars.
