Modelling the orientation of accretion discs in quasars using Hα emission

Infrared spectroscopy of the H α emission lines of a sub-sample of 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 may include combinations of classical broad-line regions of fast-moving gas clouds lying outside the quasar nucleus, and a theoretical model of emission from an optically-thick, rotating outer accretion disk. All bar one of the nineteen sources were 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 which includes two simple broad lines. Thus the best explanation for the complexity required to fit the broad H α lines in this sample is optical emission from an accretion disk in addition to the H α emission from 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. Deprojecting radio sources on the assumption of jets emerging perpendicular to the accretion disk gives rough agreement with expectations of radio source models. The distribution in disk angles is broadly consistent with models in which a Doppler boosted core can contribute to the chances of observing a source at low inclination to the line of sight, and in which the radio jets expand at constant speed up to a size of � 1 Mpc. A weak correlation was 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 first suggested by Lawrence (1991) in which the opening angle of the torus widens with increasing radio luminosity. The highest accretion disk angle measured is 48 ◦ , consistent with the opening angle predicted for