Active galactic nuclei. I. Observations and fundamental interpretations.

Part I of this review summarizes astronomical observations of active galactic nuclei made in all bands of the electromagnetic spectrum. The objects receiving the greatest attention are quasars, radio galaxies, Seyfert galaxies and BL Lacertae objects, although other types of active galaxies are also discussed. Basic models for the powerhouses of these active galaxies are considered, with an emphasis placed on energy production efficiency, compactness and variability. The leading theories, which involve accretion of plasma onto supermassive black holes, are presented, and some of the evidence favoring these models over their rivals is discussed. The second part of this review will discuss the spectra produced by the various models in more detail, along with the production and collimation of jets in active galaxies. Part II will also examine in detail various schemes that purport to explain the variety of active galaxies in a unified fashion.