Unified Schemes for Radio-Loud Active Galactic Nuclei

Unified Schemes for Radio-Loud Active Galactic Nuclei

9 Jun 1995 | C. Megan Urry and Paolo Padovani
The appearance of active galactic nuclei (AGN) is strongly dependent on orientation, leading to current classification schemes that are dominated by random pointing directions rather than physical properties. This review focuses on unifying radio-loud AGN, including radio galaxies, quasars, and blazars. It discusses the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming, and outlines two plausible unified schemes: one linking high-luminosity sources (quasars and luminous radio galaxies) and another linking low-luminosity sources (BL Lac objects and less luminous radio galaxies). The review also analyzes the connections between low- and high-luminosity radio-loud AGN, concludes that they are likely powered by similar physical processes, and addresses potential difficulties with unification. Finally, it discusses possible complications to unified schemes and identifies ten pressing questions in the field.The appearance of active galactic nuclei (AGN) is strongly dependent on orientation, leading to current classification schemes that are dominated by random pointing directions rather than physical properties. This review focuses on unifying radio-loud AGN, including radio galaxies, quasars, and blazars. It discusses the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming, and outlines two plausible unified schemes: one linking high-luminosity sources (quasars and luminous radio galaxies) and another linking low-luminosity sources (BL Lac objects and less luminous radio galaxies). The review also analyzes the connections between low- and high-luminosity radio-loud AGN, concludes that they are likely powered by similar physical processes, and addresses potential difficulties with unification. Finally, it discusses possible complications to unified schemes and identifies ten pressing questions in the field.
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