An edge-brightened bicone in the nuclear regions of Cygnus A

Infrared-imaging observations that span the wavelength range of 0.8-2.35 mu m have been obtained for the archetypal powerful radio galaxy Cygnus A usi ng the Near-Infrared Camera and Multiobject Spectrograph on beard the Hubbl e Space Telescope. At 2.25 mu m, the images are dominated by the presence o f a nuclear point source (FWHM < 0 ".21), whose flux is a factor of similar to 4 times less than the limits deduced from previous groundbased studies. The observations also reveal an edge-brightened biconical structure center ed on the point source, which is strikingly similar to those observed aroun d young stellar objects. The high polarization and orientation of the bicon e relative to the radio axis lead us to conclude that it is an illuminated structure, while the edge brightening provides evidence that the bicone is defined as much by outflows in the nuclear regions as by the polar diagram of the illuminating quasar radiation field. A further implication of our ob servations is that not all of the anisotropy in the nuclear radiation field is caused by extinction on a scale less than 100 pc in the torus; some of the anisotropy must be generated by absorption and scattering in the dust l ane on a 1 kpc scale.