The physical properties of the gas in AGNs depend on its location, density, column density, and composition. These determine the ionization parameter, the relative importance of gravity and radiation pressure force, the dust content, the gas velocity, and more. In this chapter, we consider several possible locations around the central BH and the gas and dust properties in each.

The broad-line region

Consider large-column-density (~ 1023 cm−2), high-density (~ 1010 cm−3) clouds situated around an accreting BH with L/LEdd~ 0.1, at alocation where L/4πr2≃ 109 erg s−1 cm−2. For a very luminous AGN, this is at about 0.1–1 pc from the BH. Assume also that the clouds can survive over many dynamical times either because they are confined or because they are the extensions of large self-gravitating bodies such as stars. For large enough column density clouds, the system is bound because gravity completely dominates over radiation pressure force. The typical Keplerian velocity at this location is ~ 3000 km s−1, which will be reflected in the widths of the emitted lines. We also assume a global (4π) covering factor of order 0.1. This means that we can neglect the effect of the radiation emitted by one cloud on its neighbors.

The above physical properties result in Uhydrogen ~ 10−2. This means that only the illuminated surfaces of the clouds are highly ionized. The most abundant ions in the ionized parts are He II–III, 0 IV–VI, C III–IV, and so on. The strongest predicted emission lines are, therefore, Hα,Lyα,C IV λ1549, and O VI λ1035.