Highly polarized structures in the near-nuclear regions of Cygnus A: intrinsic anisotropy within the cones?

We present near-infrared imaging polarimetry observations of the nucleus of Cygnus A (z = 0.0567), taken with the NICMOS camera of the Hubble Space Te lescope (HST) at a wavelength of 2.0 mu m. These maps reveal a highly colli mated region of polarized emission straddling the nucleus and extending to a radius of 1.2 arcsec. Remarkably, this feature coincides with one, but on ly one, limb of the edge-brightened bicone structure seen in the total inte nsity image. The high degree (P-k similar to 25 per cent) and orientation o f the extended polarization feature are consistent with a scattering origin . Most plausibly, the detection of polarization along only one limb of the bicone is a consequence of intrinsic anisotropy of the near-infrared contin uum within the radiation cones, with the direction of maximum intensity of the near-infrared radiation field significantly displaced from the directio n of the radio axis. The unresolved nuclear core source is also highly pola rized (P-k > 28 per cent), with a position angle close to perpendicular to the radio axis. Given that this high degree of nuclear polarization can onl y be explained in terms of dichroic extinction if the dichroic mechanism is unusually efficient in Cygnus A, it is more likely that the nuclear polari zation is caused by the scattering of nuclear light in an unresolved scatte ring region close to the active galactic nucleus. In this case, the flux of the core source in the K band is dominated by scattered rather than transm itted quasar light, and previous extinction estimates based on K-band photo metry of the core substantially underestimate the true nuclear extinction.