Active Galactic Nuclei: Basic Physics and Main Components

This review gives a brief introduction to the physics and the structure of active galactic nuclei (AGN). The main topics that are being discussed are general accretion, the structure and the spectrum of various types of accretion disks, photoionization and recombination, energy balance and gas temperature, the spectrum of ionized gas including the effect of dust, and the dynamics of ionized gas in a strong radiation field. These principles are then applied to various AGN components including the broad and the narrow line regions, the ionized X-ray gas, the torus and the central disk. 1 Terminology, Classification and Abbreviations The study of active galactic nuclei (AGN) has produced many new terms and abbreviations. Only the most common ones will be used in this review, e.g., the narrow line region (NLR), the broad line region (BLR), the highly ionized gas (HIG) and the black hole (BH). I shall avoid, as much as possible, using names such as Seyfert 1s, Seyfert 2s and LINERs. Instead I shall refer to type-I AGN (those objects showing broad, strong optical-UV emission lines in their spectrum), and type-II AGN (those showing prominent narrow emission lines, very faint, if any, broad lines, and a large X-ray obscuring column).