Modelling the orientation of accretion disks in quasars using H-alpha
  emission
  Infrared spectroscopy of the H-alpha emission lines of a sub-sample of 19
high-redshift (0.8 < z < 2.3) Molonglo quasars, selected at 408 MHz, is
presented. These emission lines are fitted with composite models of broad and
narrow emission, which include combinations of classical broad-line regions of
fast-moving gas clouds lying outside the quasar nucleus, and/or a theoretical
model of emission from an optically-thick, flattened, rotating accretion disk.
All bar one of the nineteen sources are found to have emission consistent with
the presence of an optically-emitting accretion disk, with the exception
appearing to display complex emission including at least three broad
components. Ten of the quasars have strong Bayesian evidence for broad-line
emission arising from an accretion disk together with a standard broad-line
region, selected in preference to a model with two simple broad lines. Thus the
best explanation for the complexity required to fit the broad H-alpha lines in
this sample is optical emission from an accretion disk in addition to a region
of fast-moving clouds. We derive estimates of the angle between the rotation
axis of the accretion disk and the line of sight. A weak correlation is found
between the accretion disk angle and the logarithm of the low-frequency radio
luminosity. This is direct, albeit tenuous, evidence for the receding torus
model. Velocity shifts of the broad H-alpha components are analysed and the
results found to be consistent with a two-component model comprising one
single-peaked broad line emitted at the same redshift as the narrow lines, and
emission from an accretion disk which appears to be preferentially redshifted
with respect to the narrow lines for high-redshift sources and blueshifted
relative to the narrow lines for low-redshift sources.
