Silicate Dust in Active Galactic Nuclei
  The unification theory of active galactic nuclei (AGNs) hypothesizes that all
AGNs are surrounded by an anisotropic dust torus and are essentially the same
objects but viewed from different angles. However, little is known about the
dust which plays a central role in the unification theory. There are
suggestions that the AGN dust extinction law appreciably differs from that of
the Galaxy. Also, the silicate emission features observed in type 1 AGNs appear
anomalous (i.e., their peak wavelengths and widths differ considerably from
that of the Galaxy). In this work, we explore the dust properties of 147 AGNs
of various types at redshifts z<0.5, with special attention paid to 93 AGNs
which exhibit the 9.7 and 18 $\mu$m silicate emission features. We model their
silicate emission spectra obtained with the Infrared Spectrograph aboard the
Spitzer Space Telescope. We find that 60/93 of the observed spectra can be well
explained with "astronomical silicate", while the remaining sources favor
amorphous olivine or pyroxene. Most notably, all sources require the dust to be
$\mu$m-sized (with a typical size of ~1.5$\pm$0.1 $\mu$m), much larger than
sub-$\mu$m-sized Galactic interstellar grains, implying a flat or "gray"
extinction law for AGNs. We also find that, while the 9.7 $\mu$m emission
feature arises predominantly from warm silicate dust of temperature T~270 K,
the ~5--8 $\mu$m continuum emission is mostly from carbon dust of T~640 K.
Finally, the correlations between the dust properties (e.g., mass, temperature)
and the AGN properties (e.g., luminosity, black hole mass) have also been
investigated.
