Title: Locating the silicate emission in NGC4151
Abstract: The unified model of active galactic nuclei (AGN), in which orientation-dependent obscuration accounts for the differences between active galaxies of types 1 and 2, predicts a 10 micron silicate absorption feature in type 2 objects and strong silicate emission from the torus in the type 1s. The silicate feature is indeed observed in absorption, albeit weakly, in many type 2 AGN, but until recently the corresponding emission feature had only been securely detected in a single type 1 Seyfert galaxy. This has posed a great problem for models attempting to explain and predict the IR emission from the dusty AGN torus. In recent months, silicate emission features have been found in Spitzer spectra of several type 1 objects, ostensibly in agreement with the predictions of the unified model. However, the apertures used for that work span hundreds if not thousands of parsecs on the galaxies, and the derived temperatures are much cooler than predicted for the surface of the torus that is directly exposed to the AGN continuum. Thus, it is still not known whether the emission feature arises in the torus itself or in extended dust of the type shown by ground-based mid-IR imaging to exist in many AGN. To distinguish between these possibilities we therefore propose a Michelle spectroscopic study of NGC4151, a bright type 1.5 AGN whose Spitzer spectrum shows a silicate feature in emission, and at 13 Mpc (1"=67 pc) the closest such galaxy yet discovered.
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