On the anomalous silicate emission features of active galactic nuclei: a possible interpretation based on porous dust

The recent Spitzer detections of the 9.7 mu m Si-O silicate emission in type 1 active galactic nuclei (AGN) provide support for the AGN unification scheme. The properties of the silicate dust are of key importance to understand the physical, chemical and evolutionary properties of the obscuring dusty torus around the AGN. Compared to that of the Galactic interstellar medium (ISM), the 10 mu m silicate emission profile of type 1 AGN is broadened and has a clear shift of peak position to longer wavelengths. In literature, this is generally interpreted as an indication of the deviations of the silicate composition, size and degree of crystallization of the AGN from that of the Galactic ISM. In this Letter, we show that the observed peak shift and profile broadening of the 9.7 mu m silicate emission feature can be explained in terms of porous composite dust consisting of ordinary interstellar amorphous silicate, amorphous carbon and vacuum. Porous dust is naturally expected in the dense circumnuclear region around the AGN, as a consequence of grain coagulation.