A complete census of silicate features in the mid-infrared spectra of
  active galaxies
  We present a comprehensive study of the silicate features at 9.7 and 18
micron of a sample of almost 800 active galactic nuclei (AGN) with available
spectra from the Spitzer InfraRed Spectrograph (IRS). We measure the strength
of the silicate feature at 9.7 micron, S9.7, before and after subtracting the
host galaxy emission from the IRS spectra. The numbers of type 1 and 2 AGN with
the feature in emission increase by 20 and 50%, respectively, once the host
galaxy is removed, while 35% of objects with the feature originally in
absorption exhibit it in even deeper absorption. The peak of S9.7, lambda_peak,
has a bimodal distribution when the feature is in emission, with about 65% of
the cases showing lambda_peak > 10.2 micron. Silicates can appear in emission
in objects with mid-infrared (MIR) luminosity spanning over six orders of
magnitude. The derived distributions of the strength of the silicate features
at 9.7 and 18 micron provide a solid test bed for modeling the dust
distribution in AGN. Clumpiness is needed in order to produce absorption
features in unobscured AGN and can also cause the silicates to be in absorption
at 9.7 micron and in emission at 18 micron in type 1 sources. We find the
`cosmic' silicates of Ossenkopf et al. to be more consistent with the
observations than Draine's `astronomical' silicates. Finally, we discuss the
possibility of a foreground absorber to explain the deep silicate absorption
features in the MIR spectra of some type 2 AGN.
