Unified Schemes for Radio-Loud Active Galactic Nuclei

The appearance of active galactic nuclei (AGN) depends so strongly onorientation that our current classification schemes are dominated by randompointing directions instead of more interesting physical properties. Light fromthe centers of many AGN is obscured by optically thick circumnuclear matter andin radio-loud AGN, bipolar jets emanating from the nucleus emit light that isrelativistically beamed along the jet axes. Understanding the origin andmagnitude of radiation anisotropies in AGN allows us to unify different classesof AGN; that is, to identify each single, underlying AGN type that gives riseto different classes through different orientations. This review describes the unification of radio-loud AGN, which include radiogalaxies, quasars, and blazars. We describe the classification and propertiesof AGN and summarize the evidence for anisotropic emission. We outline the twomost plausible unified schemes for radio-loud AGN, one linking quasars andluminous radio galaxies and another linking BL~Lac objects and less luminousradio galaxies. Using the formalism appropriate to samples biased byrelativistic beaming, we show the population statistics for two schemes are inaccordance with available data. We analyze the possible connections betweenlow- and high-luminosity radio-loud AGN. We review potential difficulties withunification and conclude that none currently constitutes a serious problem. Wediscuss likely complications to unified schemes that are suggested by realisticphysical considerations; these will be important to consider when morecomprehensive data for larger complete samples become available. We concludewith a list of the ten questions we believe are the most pressing in thisfield.