The Detection of Silicate Emission from Quasars at 10 and 18 Microns
  We report the spectroscopic detection of silicate emission at 10 and 18 microns in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is consistent with the unification model for Active Galactic Nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 microns and from 0.20 to 0.79 times the continuum at 18 microns. The silicate grain temperatures inferred from the ratio of 18-to-10 micron silicate features under the assumption of optically thin emission range from 140 to 220K. 
Lei Hao (1), H. W. W. Spoon (1), G. C. Sloan (1), J. A. Marshall (1),   L. Armus (2), A. G. G. M. Tielens (3), B. Sargent (4...