8.2.5 Selection Effects in the Identification of Quasars
If quasar unification schemes are correct, all quasars are oriented at a preferred angle from which the broad-line region is visible. This undoubtedly has strong effects on other quasar properties. For example, it could explain why no Ly edges are seen in the ultraviolet spectra of quasars (Koratkar et al. 1992; Kinney 1994) even though they are expected from accretion disks, an important element in the current AGN paradigm (Fig. 1). That is, assuming that the axes of accretion disks and obscuring matter coincide, no edge-on disks would be seen and so no Ly edges should be seen.
Other secondary effects are also possible. If the accretion disks in quasars are thick, as expected for high-luminosity AGN, then their radiation patterns can be anisotropic independent of the obscuring torus (Madau 1988). Because its radiation pattern is narrower than the opening angle of the obscuring torus, the thick disk can by itself introduce mild selection effects (Urry et al. 1991b) wherein quasars are more face-on than implied by their emission line properties (assuming the axes of the thick disk and torus are aligned). In that case, the optical-through-ultraviolet continuum emission from quasars will appear bluer and more luminous than if it were emitted isotropically.