Dust in active galactic nuclei
Abstract
Dust is the cornerstone of the unification theory of active galactic nuclei (AGNs). This theory proposes that all AGNs are essentially "born equal": all types of AGNs are surrounded by an optically thick dust torus and are basically the same object but viewed from different lines of sight. The large diversity in the observational properties of AGNs is simply caused by the viewing-angle-dependent obscuration of the nucleus: those viewed face-on are unobscured (allowing for a direct view of their nuclei) and recognized as "type 1" AGNs, while those viewed edge-on are "type 2" AGNs with most of their central engine and broad line regions being hidden by the obscuring dust. However, little is known about the dust in the circumnuclear torus of AGNs. There is evidence suggesting that the size and composition of the dust in AGNs may differ substantially from that of the Galactic interstellar dust, as reflected by the flat "gray" extinction curves, the 2175 Å extinction bump is not (or at least rarely) seen in AGNs, and the anomalous silicate emission or absorption features observed respectively in type1 and type 2 AGNs. This review summarizes our current understanding of the extinction and infrared emission properties of the circumnuclear dust in AGNs as well as the inferred dust composition and size distribution.
