The 9.7 micron silicate dust absorption toward the Cygnus A nucleus and the inferred location of the obscuring dust

We report the detection of a 9.7 mu m 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 (A(V) similar to 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 mu m 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 (A(V) > 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.