Astronomy and Astrophysics – Astrophysics
Scientific paper
2000-01-18
Astronomy and Astrophysics
Astrophysics
17 pages, 2 figures, accepted for publication in ApJ
Scientific paper
10.1086/308893
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 ($A_{\rm V}$ $\sim$ 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 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 ($A_{\rm V}$ $>$ 50 mag) by a dusty torus with an inner radius of $<$10 pc, and not by $>$ a few 100 pc scale dust in the host galaxy.
Imanishi Masatoshi
Ueno Shiro
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