Cygnus A resolved: long-slit spectroscopy of the extended emission-line system

High spatial resolution, long-slit spectra are used to investigate the reddening, kinematics and ionization of the extended ionized gas in the powerful radio galaxy Cygnus A. Measurements of the reddening using the Hα/Hβ, Hγ/Hβ recombination line ratios show a well-defined maximum along the radio axis, roughly coincident with the radio nucleus, and hence support the dust-lane model for the optical double structure. The emission-line kinematics are generally characteristic of the systems of filaments found in central cluster galaxies with massive X-ray haloes, but we also identify more extreme components associated with the radio axis and the radio nucleus. These components, with linewidths up to 600 km s-1 (FWHM), point to the disturbing influence of the radio plasma and the active nucleus. The general emission-line spectrum and the line-ratio variations across the source challenge conventional power-law photoionization models. In particular, the [NII] λ6584/Hα ratio is significantly larger than predicted by models with solar abundances and, together with [SII] λλ(6717 + 6731)/Hα, remains roughly constant across the nebula along PA 105° and PA 15°. In contrast, line ratios involving [O III] λ5007 show distinct variations along the direction perpendicular to the radio axis (PA 15°), in a pattern consistent with photoionization by an anisotropic radiation field emanating from a central source. It is likely that the N/O abundance ratio is significantly enhanced (N/O ∼ 4 × solar), and that a distributed source of ionization is required in addition to the photoionization by the active nucleus. Consequently, while the ionizing radiation field appears to be anisotropic, the uncertainty as to the ionization mechanism means that we can place only a lower limit on the power of the central continuum source.