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Accepted for publication in the Astrophysical Journal Letters Silicate emissions in active galaxies- From LINERs to QSOs

By E. Sturm, M. Schweitzer, D. Lutz, A. Contursi, R. Genzel, M. D. Lehnert, L. J. Tacconi, S. Veilleux, D. S. Rupke, D. -c. Kim, A. Sternberg, D. Maoz, S. Lord and J. Mazzarella


We report the first detection of ∼10 and ∼18µm silicate dust emissions in a low-luminosity active galactic nucleus (AGN), obtained in Spitzer-IRS 7-37µm spectroscopy of the Type 1 LINER galaxy NGC3998. Silicate emissions in AGN have only recently been detected in several quasars. Our detection counters suggestions that silicate emissions are present only in the most luminous AGN. The silicate features may be signatures of a dusty “obscuring torus ” viewed face-on as postulated for Type 1 AGN. However, the apparently cool (∼200 K) dust is inconsistent with theoretical expectations of much hotter torus walls. Furthermore, not all Type 1 objects are silicate emission sources. Alternatively, the – 2 – silicate emission may originate in dust not directly associated with a torus. We find that the long-wavelength (�20µm) tail of the emission in NGC3998 is significantly weaker than in the sample of bright QSOs recently presented by Hao et al. The 10µm profile in our NGC3998 spectrum is inconsistent with “standard” silicate ISM dust. This may indicate differences in the dust composition, grain size distribution, or degree of crystallization. The differences between NGC3998, QSOs, and Galactic templates suggest that there are significant environmental variations

Year: 2005
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