Abstract
The “intermediate zone” is the several decades in radius between the broad and narrow emission line regions in active galactic nuclei. As Antonucci and Miller (1985) first discovered, it comprises two basic structures: an obscuring torus, and a reflection region which fills the hole of the torus and extends some ways above its top. Recent theoretical work, reviewed here, has concentrated on understanding both the thermal/ionization state, and the dynamical state of both regions. The torus is probably composed of a large number of dusty, molecular clouds of large column density and unusually high temperature and ionization fraction, while the reflection region is hot and highly ionized. Spectroscopic diagnostics exist for both. The key problem, still essentially unsolved, in the dynamics of the torus is its geometrical thickness. The reflection region is probably a pressure-driven transonic wind whose source is material evaporated by photoionization from the inner surface of the torus, but a small fraction of it may actually be accreting onto the nucleus.
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