Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-08-29
Astronomy and Astrophysics
Astrophysics
6 pages, 4 figures; Mon. Not. Roy. Astron. Soc. Lett., in press
Scientific paper
10.1111/j.1745-3933.2008.00553.x
The recent Spitzer detections of the 9.7 micron Si--O silicate emission in type 1 AGNs provide support for the AGN unification scheme. The properties of the silicate dust are of key importance to understanding the physical, chemical and evolutionary properties of the obscuring dusty torus around AGNs. Compared to that of the Galactic interstellar medium (ISM), the 10 micron silicate emission profile of type 1 AGNs 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 AGNs from that of the Galactic ISM. In this Letter we show that the observed peak shift and profile broadening of the 9.7 micron 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 AGNs, as a consequence of grain coagulation.
Li Aigen
Li Mo-Ping
Shi Q. J.
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