A systematic study of silicate absorption features in heavily obscured
  AGNs observed by Spitzer/IRS
  Heavily obscured active galactic nuclei (AGNs) are known to show deep
silicate absorption features in the mid-infrared (IR) wavelength range of
10--20~$\mu$m. The detailed profiles of the features reflect the properties of
silicate dust, which are likely to include information on AGN activities
obscured by large amounts of dust. In order to reveal AGN activities obscured
by large amounts of dust, we select 115 mid-IR spectra of heavily obscured AGNs
observed by Spitzer/IRS, and systematically analyze the composition of silicate
dust by spectral fitting using the 10~$\mu$m amorphous and 23~$\mu$m
crystalline bands. We find that the main component of the silicate dust
obscuring AGNs is amorphous olivine, the median mass column density of which is
one order of magnitude higher than those of the minor components of amorphous
pyroxene and crystalline forsterite. The median mass fraction of the amorphous
pyroxene, $\sim$2\%, is significantly lower than that of the diffuse
interstellar medium (ISM) dust in our Galaxy, while the median mass fraction of
the crystalline forsterite, $\sim$6\%, is higher than that of the diffuse ISM
dust. We also find that the mass fractions of the amorphous pyroxene and the
crystalline forsterite positively correlate with each other. The low mass
fraction of the amorphous pyroxene suggests that the obscuring silicate dust is
newly formed, originating from starburst activities. The relatively high mass
fraction of crystalline forsterite implies that the silicate dust is processed
in the high temperature environment close to the nucleus and transported to
outer cooler regions by molecular outflows. The positive correlation between
the mass fractions can be naturally explained considering that amorphous
pyroxene is transformed from crystalline forsterite by ion bombardments.
