Scattered Nuclear Continuum and Broad Hα in Cygnus A

Abstract

We have discovered scattered broad Balmer emission lines in the spectrum of Cygnus A, using the Keck II Telescope. Broad Hα appears in polarized flux from components on either side of the nucleus and, to a lesser extent, in the nucleus. The full-width at half-maximum of broad Hα is 26,000 km s-1, comparable to the widest emission lines seen in broad-line radio galaxies. Scattered active galactic nucleus light provides a significant contribution to the total flux at 3800 A (rest) of the western component, where the polarization rises to 16%. The spatially integrated flux of Cygnus A at 5500 A can be decomposed into an elliptical galaxy fraction (Fg = 0.70), a highly polarized blue component (FC1 = 0.15), a less polarized red component (FC = 0.09), and a contribution from the nebular continuum (0.06). Imaging polarimetry shows a double fan of polarization vectors with circular symmetry that corresponds to the ionization cone seen in Hubble Space Telescope images. Our results are consistent with scattering of light from a hidden quasar of modest luminosity by an extended, dusty narrow-line region.