The 9.7 Micron Silicate Dust Absorption toward the Cygnus A Nucleus and the Inferred Location of the Obscuring Dust

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Abstract
We report the detection of a 9.7 μm silicate dust absorption feature toward the Cygnus A nucleus. Its optical depth is, however, significantly smaller than that expected from the dust extinction toward the background L-band emission region (AV ~ 150 mag). We argue that the most likely explanation for the small optical depth is that the obscuring dust exists so close to the central engine that a temperature gradient occurs. Our calculation confirms that the small optical depth and the spectral energy distribution at 3-30 μm can be quantitatively reproduced by this explanation. Combining this picture with the huge active galactic nucleus (AGN) luminosity, the emission properties of Cygnus A are consistent with those of a type 2 quasar, that is, a highly luminous AGN that is highly obscured (AV > 50 mag) by a dusty torus with an inner radius of less than 10 pc and not by greater than a few hundred parsec scale dust in the host galaxy.
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