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 Telescope (HST) at a wavelength of 2.0 μm. These maps reveal a highly collimated region of polarized emission straddling the nucleus and extending to a radius of 1.2 arcsec. Remarkably, this feature coincides with one, but only one, limb of the edge-brightened bicone structure seen in the total intensity image. The high degree (Pk∼25 per cent) and orientation of 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 continuum within the radiation cones, with the direction of maximum intensity of the near-infrared radiation field significantly displaced from the direction of the radio axis. The unresolved nuclear core source is also highly polarized (Pk>28 per cent), with a position angle close to perpendicular to the radio axis. Given that this high degree of nuclear polarization can only be explained in terms of dichroic extinction if the dichroic mechanism is unusually efficient in Cygnus A, it is more likely that the nuclear polarization is caused by the scattering of nuclear light in an unresolved scattering 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 transmitted quasar light, and previous extinction estimates based on K-band photometry of the core substantially underestimate the true nuclear extinction.