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 (see e.g. Antonucci 1993; Urry & Padovani 1995). The large diversity in the observational properties of AGNs (e.g. optical emission-line widths and X-ray spectral slopes) 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.
Apparently, key factors in understanding the structure and nature of AGNs are determining the geometry of the nuclear obscuring torus around the central engine and the obscuration (i.e. extinction, a combination of absorption and scattering) properties of the circumnuclear dust. An accurate knowledge of the dust extinction properties is also required to correct for the dust obscuration in order to recover the intrinsic optical/ultraviolet (UV) spectrum of the nucleus from the observed spectrum and to probe the physical conditions of the dust-enshrouded gas close to the nucleus.
The presence of an obscuring dust torus around the central engine was first indirectly indicated by the spectropolarimetric detection of broad permitted emission lines (characteristic of type 1 AGNs) scattered into our line of sight by free electrons located above or below the dust torus in a number of type 2 AGNs (e.g. see Heisler et al. 1997, Tran 2003). Direct evidence for the presence of a dust torus is provided by infrared (IR) observations. The circumnuclear dust absorbs the AGN illumination and reradiates the absorbed energy in the IR. The IR emission at wavelengths longward of > 1 µm accounts for at least 50% of the bolometric luminosity of type 2 AGNs. For type 1 AGNs, ~ 10% of the bolometric luminosity is emitted in the IR (e.g. see Fig. 13.7 of Osterbrock & Ferland 2006). A near-IR "bump" (excess emission above the ~ 2-10 µm continuum), generally attributed to hot dust with temperatures around ~ 1200-1500 K (near the sublimation temperatures of silicate and graphite grains), is seen in a few type 1 AGNs (Barvainis 1987; Rodríguez-Ardila & Mazzalay 2006). Direct imaging at near- and mid-IR wavelengths has been performed for several AGNs and provides constraints on the size and structure of the circumnuclear dust torus (e.g. see Jaffe et al. 2004, Elitzur 2006). Spectroscopically, the 10 µm silicate absorption feature (see Section 3.3) and the 3.4 µm aliphatic hydrocarbon absorption feature (see Section 3.2) are widely seen in heavily obscured type 2 AGNs; in contrast, the 10 µm silicate emission feature has recently been detected in a number of type 1 AGNs (see Section 3.3).
To properly interpret the observed IR continuum emission and spectroscopy as well as the IR images of AGNs, it requires a good understanding of the absorption and emission properties of the circumnuclear dust. To this end, one needs to know the composition, size, and morphology of the dust - with this knowledge, one can use Mie theory (for spherical dust) to calculate the absorption and scattering cross sections of the dust from X-ray to far-IR wavelengths, and then calculate its UV/optical/near-IR obscuration as a function of wavelength, and derive the dust thermal equilibrium temperature (based on the energy balance between absorption and emission) as well as its IR emission spectrum. This will allow us to correct for dust obscuration and constrain the circumnuclear structure through modeling the observed IR emission and images. The former is essential for interpreting the obscured UV/optical emission lines and probing the physical conditions of the central regions; the latter is critical to our understanding of the growth of the central supermassive black hole.
However, little is known about the dust in the circumnuclear torus of AGNs. Even our knowledge of the best-studied dust - the Milky Way interstellar dust - is very limited. In this review, I will take a comparative study of the extinction and IR emission as well as the UV/IR spectroscopic properties and the inferred composition, size and morphology of the dust in AGNs and the dust in the interstellar medium (ISM) of the Milky Way and other galaxies.