Unified Schemes for Radio-Loud Active Galactic Nuclei

Abstract

The appearance of active galactic nuclei (AGN) depends so strongly on\norientation that our current classification schemes are dominated by random\npointing directions instead of more interesting physical properties. Light from\nthe centers of many AGN is obscured by optically thick circumnuclear matter and\nin radio-loud AGN, bipolar jets emanating from the nucleus emit light that is\nrelativistically beamed along the jet axes. Understanding the origin and\nmagnitude of radiation anisotropies in AGN allows us to unify different classes\nof AGN; that is, to identify each single, underlying AGN type that gives rise\nto different classes through different orientations.\n This review describes the unification of radio-loud AGN, which include radio\ngalaxies, quasars, and blazars. We describe the classification and properties\nof AGN and summarize the evidence for anisotropic emission. We outline the two\nmost plausible unified schemes for radio-loud AGN, one linking quasars and\nluminous radio galaxies and another linking BL~Lac objects and less luminous\nradio galaxies. Using the formalism appropriate to samples biased by\nrelativistic beaming, we show the population statistics for two schemes are in\naccordance with available data. We analyze the possible connections between\nlow- and high-luminosity radio-loud AGN. We review potential difficulties with\nunification and conclude that none currently constitutes a serious problem. We\ndiscuss likely complications to unified schemes that are suggested by realistic\nphysical considerations; these will be important to consider when more\ncomprehensive data for larger complete samples become available. We conclude\nwith a list of the ten questions we believe are the most pressing in this\nfield.\n