4.7. Unified Models
After the discovery of QSOs, the widely different appearances of different AGN became appreciated. The question arose, what aspects of this diversity might result from the observer's location relative to the AGN? A basic division was between radio loud and radio quiet objects. Since the extended radio sources radiate fairly isotropically, their presence or absence could not be attributed to orientation. Furthermore, radio loud objects seemed to be associated with elliptical galaxies, and radio quiet AGN with spiral galaxies. The huge range of luminosities from Seyferts to QSOs clearly was largely intrinsic. However, some aspects could be a function of orientation. Blandford and Rees (1978) proposed that BL Lac objects were radio galaxies viewed down the axis of a relativistic jet. Relativistic beaming caused the nonthermal continuum to be very bright when so viewed, and the emission lines (emitted isotropically) would be weak in comparison. The same object, viewed from the side, would have normal emission-line equivalent widths, and the radio structure would be dominated by the extended lobes rather than the core.
A key breakthrough occurred as a result of advances in the techniques of spectropolarimetry. Rowan-Robinson (1977) had raised the possibility that the BLR of Seyfert 2 galaxies was obscured by dust, rather than being truly absent. Using a sensitive spectropolarimeter on the 120-inch Shane telescope at Lick Observatory, Antonucci and Miller (1985) found that the polarized flux of NGC 1068, the prototype Seyfert 2, had the appearance of a normal Seyfert 1 spectrum. This was interpreted in terms of a BLR and central continuum source obscured from direct view by an opaque, dusty torus. Electron scattering material above the nucleus near the axis of the torus scattered the nuclear light to the observer, polarizing it in the process. This allowed Seyfert 2's to have a detectable but unreddened continuum. However, the broad lines had escaped notice because the scattered light was feeble compared with the narrow lines from the NLR, which was outside the presumed obscuring torus. The same object, viewed face on, would be a Seyfert 1. Such a picture had also been proposed by Antonucci (1984) for the broad line radio galaxy 3C 234. Various forms of toroidal geometry had been anticipated by Osterbrock (1978) and others, and the idea received support from the discovery of "ionization cones" in the nuclei of some AGN (Pogge 1988). Orientation indicators were developed involving the ratio of the core and extended radio luminosities (Orr and Browne 1982; Wills and Browne 1986). The concepts of a beamed nonthermal continuum and an obscuring, equatorial torus remain fundamental to current efforts to unify AGN. Consideration of the obscuring torus supports the idea that the X-ray background is produced mostly by AGN (Setti and Woltjer 1989).