Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry toward Galactic Nuclei

The silicate cross section peak near 10um produces emission and absorptionfeatures in the spectra of dusty galactic nuclei observed with the SpitzerSpace Telescope. Especially in ultraluminous infrared galaxies, the observedabsorption feature can be extremely deep, as IRAS 08572+3915 illustrates. Aforeground screen of obscuration cannot reproduce this observed feature, evenat large optical depth. Instead, the deep absorption requires a nuclear sourceto be deeply embedded in a smooth distribution of material that is bothgeometrically and optically thick. In contrast, a clumpy medium can produceonly shallow absorption or emission, which are characteristic ofoptically-identified active galactic nuclei. In general, the geometry of thedusty region and the total optical depth, rather than the grain composition orheating spectrum, determine the silicate feature's observable properties. Theapparent optical depth calculated from the ratio of line to continuum emissiongenerally fails to accurately measure the true optical depth. The obscuringgeometry, not the nature of the embedded source, also determines the far-IRspectral shape.Comment: To appear in ApJ