Unification of AGNs
There is now ample evidence for partial unification of (a) Seyfert 1 and Seyfert 2 galaxies, in terms of obscuring tori and orientation effects; (b) steep-spectrum (lobe dominated) radio quasars and flat-spectrum (core dominated) radio quasars, in terms of relativistic beaming and orientation effects; and (c) blazars, radio-loud quasars and broad-line radio galaxies, and narrow-line radio galaxies, in terms of obscuring tori, relativistic beaming, and orientation effects. Variants or subsets of these basic sets have also been proposed — e.g., BL Lac objects and Fanaroff-Riley (1974) Class I galaxies. Much of the observational evidence was discussed at this conference, primarily by Bob Fosbury, Clive Tadhunter, Neal Jackson, Thaisa Storchi-Bergmann, Paul Alexander, Paolo Padovani, Dave Axon, and Alec Boksenberg, as well as in several posters; it involves spectropolarimetry, superluminal motion, projected sizes, relative numbers of different objects, lobe depolarization asymmetry, the inverse-Compton limit, ionization cones, variability, and other phenomena. A reasonable conclusion is that unification schemes must be correct at least to some degree.
It is not so clear, however, that the strong forms of the unification schemes are valid. Specifically, let me focus on the unification of Seyfert 1 and Seyfert 2 galaxies (Antonucci & Miller 1985). The strong form states that these are exactly the same objects, simply viewed from different directions; no other factors (such as differences in the thickness or opening angle of the obscuring torus) are involved.