Strong anisotropy in AGN is well established (Secs. 3 and Sec. 4). This means some form of unification through orientation must be valid. The first step in understanding AGN, then, is to unify the principal classes of radio-loud AGN as described in this review. The next step is to untangle the effects of cosmic evolution, which causes the luminosity, morphology, and/or number of AGN to change with time (redshift). With an ensemble of AGN for which orientation and evolutionary effects have been removed, we will finally be able to approach the more physically interesting issues of black hole mass, black hole spin, accretion rate, formation and evolution of the individual radio source, etc.
For radio-loud AGN - radio galaxies, quasars, and blazars - relativistic beaming is likely to be very important, on both small and large scales. This has an enormous effect on the observed properties. Two schemes, one unifying low-luminosity (FR I) radio galaxies with weak-lined blazars (BL Lac objects) and the other unifying high-luminosity (FR II) radio galaxies with strong-lined blazars (OVV, HPQ, FSRQ), and SSRQ, are both consistent with essentially all available data (Secs. 5, 6, and 7). There are some remaining problems or complications that require further analysis (Sec. 8) but these are far from falsifying the concept of unification.
From today's vantage point, we identify the ten most important issues for unification and ultimately for understanding AGN. That unification works quite well with very simple models means we do understand something important about AGN. As we learn more and more about radio sources, and the simplicity of present unified schemes inevitably gives way to increasing complexity, we will doubtless discover even more about what it is that we do not know. As an anonymous person (who was surely a scientist) said: The greater the sphere of our knowledge, the larger is the surface of its contact with the infinity of our ignorance. (19) In that spirit, recognizing that the questions we ask today, which we believe will lead to fundamental understanding of AGN, may not be the questions asked or answered tomorrow, we put forward the following list:
(1) Is there evidence for BL Lacs or obscured quasars in all radio galaxies?
(2) What is the relation between HBL, LBL and FSRQ?
(3) Are the observed distributions of a, R, and jet/counter-jet ratios commensurate with beaming?
(4) Is the Lorentz factor higher in the high-luminosity radio sources (quasars and FR II radio galaxies) than in low-luminosity radio sources (BL Lac objects and FR I radio galaxies)?
(5) Do FR Is have broad emission line regions?
(6) What is the relation between FR Is and FR IIs?
(7) How do jets form and propagate?
(8) What is the physical cause of the radio loud - radio quiet distinction?
(9) Where are the narrow-line (Type 2) radio-quiet quasars?
(10) What are the fundamental parameters governing the central engine, and is it powered by a black hole?
We thank many of our colleagues for helpful comments and discussions,
including Ski Antonucci, Stefi Baum, John Biretta, Howard Bond,
Sperello di Serego Alighieri, James Dunlop, Mike Eracleous, Carla Fanti,
Roberto Fanti, Bob Fosbury, Gabriele Ghisellini, Paolo Giommi,
Bob Goodrich, Gopal-Krishna, Ann Gower, Tim Heckman, John Hutchings,
Buell Jannuzi, Ken Kellermann, Ron Kollgaard, Robert Laing, Andy Lawrence,
Laura Maraschi, Raffaella Morganti, Chris O'Dea, Frazer Owen, Joe Pesce,
Eric Perlman, Rita Sambruna, Chris Simpson, Ashok Singal, John Stocke,
Steve Unwin, and Paul Wiita. We further thank
Jim Braatz, James Dunlop, John Hutchings, Raffaella Morganti, Frazer Owen,
Geoffrey Taylor, and Andrew Wilson for providing data, images, and figures;
Joe Pesce, John Godfrey, and Dave Paradise for help producing figures;
Sarah Stevens-Rayburn for library assistance;
and the Space Telescope
Science Institute, the STScI Visitor Program,
Christina Padovani, and Andrew Szymkowiak for support of the collaborative
visits which made completion of this work possible.
19 Anonymous quotation in A Short History of Astronomy, A. Berry (New York, Dover). Back.