On the 10-micron silicate feature in Active Galactic Nuclei
  The 10-micron silicate feature observed with Spitzer in active galactic
nuclei (AGN) reveals some puzzling behavior. It (1) has been detected in
emission in type 2 sources, (2) shows broad, flat-topped emission peaks shifted
toward long wavelengths in several type 1 sources, and (3) is not seen in deep
absorption in any source observed so far. We solve all three puzzles with our
clumpy dust radiative transfer formalism. (1) We present the spectral energy
distribution (SED) of SST1721+6012, the first type 2 quasar observed to show a
clear 10-mic silicate feature in emission. We constructed a large database of
clumpy torus models and performed extensive fitting of the observed SED,
constraining several of the torus parameters. We find that the source
bolometric luminosity is ~3*10^12 L_sun. Our modeling suggests that <35% of
objects with tori sharing characteristics and geometry similar to the best fit
would have their central engines obscured. This relatively low obscuration
probability can explain the clear appearance of the 10-mic emission feature in
SST1721+6012 together with its rarity among other QSO2. (2) We also fitted the
SED of PG1211+143, one of the first type 1 QSOs with a 10-mic silicate feature
in emission. Among similar sources, this QSO appears to display an unusually
broadened feature whose peak is shifted toward longer wavelengths. Although
this led to suggestions of non-standard dust chemistry in these sources, our
analysis fits such SEDs with standard galactic dust; the apparent peak shifts
arise from radiative transfer effects. (3) We find that the distribution of
silicate feature strengths among clumpy torus models closely resembles the
observed distribution, and the feature never occurs deeply absorbed. (abridged)
