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[140.11] 9.7 um Silicate Features in AGNs: the New Insight to Unification Models

Y. Shi, G. Rieke (Steward Observatory, University of Arizona), D. Hines (Space Science Institue), V. Gorjian, M. Werner, K. Cleary (Jet Propulsion Laboratory, California Institute of Technology), F. Low, P. Smith (Steward Observatory, University of Arizona), J. Bouwman (Max-Planck-Institut fur Astronomie, Germany Heidelberg, Germany)

We report IRS spectroscopic observations of 9.7 um silicate features in 83 AGNs, exhibiting a wide range of AGN types and X-ray extinction toward the central nuclei. We find that the feature-strength decreases with increasing X-ray-absorbing column density in a way that is generally consistent with unification models where the large diversity in AGN properties is due to the viewing-angle-dependent obscuration by surrounding material. However, Compton-thick AGNs do not show deep silicate absorption as expected from the correlation. Based on the above results, we suggest a model with a very dense, smooth central component (0.1-10 pc in radius) with denser clouds embedded and an outer, clumpy component (3-300 pc in radius). The inner radius of the smooth component extends to the central accretion disk to provide Compton-thick obscurations. While the smooth component is the silicate-emission region, the silicate-absorption clouds (N_H< 10²³cm^-2) located in front of the smooth component and the X-ray-absorbing clouds (N_H> 10²³cm^-2) are embedded in the smooth component. The similarity of the behavior of various types of AGN suggests that this disk geometry may be typical for AGN in general.

Support for this work was provided by NASA through contract 960785 issued by JPL/California Institute of Technology.

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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.