科研之友
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摘要
Clumpy torus models with N-0 similar to 5-15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations. The dust has standard Galactic composition, with individual cloud optical depth tau(V) similar to 30-100 at visual. The models naturally explain the observed behavior of the 10 mu m silicate feature, in particular the lack of deep absorption features in AGNs of any type, and can reproduce the weak emission feature tentatively detected in type 2 QSOs. The clouds' angular distribution must have a soft edge, e. g., Gaussian, and the radial distribution should decrease as 1/r or 1/r(2). In line with recent interferometry, the ratio of the torus outer to inner radius can be as small as similar to 5-10. The models can produce nearly isotropic IR emission together with highly anisotropic obscuration, as required by observations. Clumpiness implies that the viewing angle determines an AGN classification only probabilistically; a source can display type 1 properties even from directions close to the equatorial plane. The fraction of obscured sources depends not only on the torus angular thickness but also on the cloud number N-0, and this fraction's observed decrease with luminosity can be explained with a decrease of either parameter. X-ray obscuration, too, is probabilistic; resulting from both dusty and dust-free clouds, it might be dominated by the latter, giving rise to the observed QSOs that are X-ray obscured. Observations indicate that the torus and broad-line-emitting clouds form a seamless distribution, with the transition between the two caused by dust sublimation. Torus clouds may have been detected in the outflow component of H2O maser emission from two AGNs. Proper-motion measurements of outflow masers, especially in Circinus, are a promising method for probing the morphology and kinematics of torus clouds.
- 出版日期2008-9-20
