Paper:	HI Absorption from a Circumnuclear TORUS in the Hidden Quasar Cygnus A
Volume:	156, Highly Redshifted Radio Lines
Page:	259
Authors:	Conway, J. E.
Abstract:	We present evidence for a compact, ( 50pc scale) rotating, flattened, distribution of HI gas perpendicular to the radio axis in the hidden quasar Cygnus A (a feature we term a TORUS for `Thick Obscuration Required by Unified Schemes'). The velocity gradient we see across this distribution implies a maximum enclosed mass of about 108 Msun within 50pc radius. Such a central mass could explain the luminosity we see from Cygnus A if it accretes at 10% of its Eddington rate. Physical conditions along the line of sight are consistent with purely atomic gas explaining the absence of millimeter absorption lines. The HI absorption is seen against diffuse counterjet continuum emission of brightness temperature 5 times 106K, which may be due to free-free emission or Thompson scattering of jet synchrotron emission.

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