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This review describes the unification of radio-loud AGN, which include radio galaxies, quasars, and blazars. We describe the classification and general properties of AGN. We summarize the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming. We outline the two most plausible unified schemes for radio-loud AGN, one linking the high-luminosity sources (quasars and luminous radio galaxies) and one the low-luminosity sources (BL~Lac objects and less luminous radio galaxies). Using the formalism appropriate to samples biased by relativistic beaming, we show the population statistics for two schemes are in accordance with available data.
We analyze the possible connections between low- and high-luminosity radio-loud AGN and conclude they probably are powered by similar physical processes, at least within the relativistic jet. We review potential difficulties with unification and conclude that none currently constitutes a serious problem. We discuss likely complications to unified schemes that are suggested by realistic physical considerations; these will be important to consider when more comprehensive data for larger complete samples become available. We conclude with a list of the ten questions we believe are the most pressing in this field.
Original Version (ADS link):
Publications of the Astronomical Society of the Pacific,
1995, 107, 803. Invited Review Paper.
Text copyright (c) 1995: Astronomical Society of the Pacific