An Edge-brightened Bicone in the Nuclear Regions of Cygnus A

Infrared-imaging observations that span the wavelength range of 0.8-2.35 μm have been obtained for the archetypal powerful radio galaxy Cygnus A using the Near-Infrared Camera and Multiobject Spectrograph on board the Hubble Space Telescope. At 2.25 μm, the images are dominated by the presence of a nuclear point source (FWHM < 021), whose flux is a factor of ~4 times less than the limits deduced from previous ground-based studies. The observations also reveal an edge-brightened biconical structure centered on the point source, which is strikingly similar to those observed around young stellar objects. The high polarization and orientation of the bicone 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 observations 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 lane on a 1 kpc scale.