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

Abstract

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 ($A_{\mathrm{V}}$ $\sim$ 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 ($A_{\mathrm{V}}$ $>$ 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.

17 pages, 2 figures, accepted for publication in ApJ

References in corpus (3)

• An X-ray Spectral Survey of Radio-Loud AGN With ASCA
• New Results from a Near-Infrared Search for Hidden Broad-Line Regions in Ultraluminous Infrared Galaxies
• Are There Any Type 2 QSOs? The Case of AXJ0341.4-4453

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