Determining quasar orientation

Abstract

Since the discovery of active galactic nuclei (AGN) and their subclasses, a unification scheme of AGN has been long sought. Orientation-based unified models predict that some of the diversity within AGN subclasses can be explained by the different viewing angles of the observer. Several orientation categorizations have been suggested, but a widely applicable measure has yet to be found. Using the properties of the ultraviolet and optical broad emission lines of quasars, in particular the velocity offsets and line widths of high-ionization C IV and low-ionization Mg II lines, a correlation has been measured. It is postulated that this correlation is due to the viewing angle of the observer. Comparison with other orientation tracers shows consistency with this interpretation. Using a simulation of a wide angle disc-wind model for the broad emission line region, we successfully replicate the observed correlation with inclination. Future more detailed modelling will not only enable improved accuracy in the determination of the viewing angle to individual AGN but also substantially increase our understanding of the emitting regions of AGN.