Silicate emissions in active galaxies: From liners to QSOs

We report the first detection of similar to 10 and similar to 18 mu m silicate dust emissions in a low-luminosity active galactic nucleus (AGN), obtained in Spitzer IRS 7 - 37 mu m spectroscopy of the type 1 LINER galaxy NGC 3998. Silicate emissions in AGNs have only recently been detected in several quasars. Our detection counters suggestions that silicate emissions are present only in the most luminous AGNs. The silicate features may be signatures of a dusty obscuring torus viewed face-on as postulated for type 1 AGNs. However, the apparently cool (similar to 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 silicate emission may originate in dust not directly associated with a torus. We find that the long-wavelength (greater than or similar to 20 mu m) tail of the emission in NGC 3998 is significantly weaker than in the sample of bright QSOs recently presented by Hao et al. The 10 mu m profile in our NGC 3998 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 NGC 3998, QSOs, and Galactic templates suggest that there are significant environmental variations.