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 t he powerful radio galaxy Cygnus A. Measurements of the reddening using the Halpha/Hbeta, Hgamma/Hbeta recombination fine ratios show a well- defined maximum along the radio axis, roughly coincident with the radi o nucleus, and hence support the dust-lane model for the optical doubl e structure. The emission-line kinematics are generally characteristic of the systems of filaments found in central cluster galaxies with ma ssive X-ray haloes, but we also identify more extreme components assoc iated with the radio axis and the radio nucleus. These components, wit h linewidths up to 600 km s-1 (FWHM), point to the disturbing influenc e of the radio plasma and the active nucleus. The general emission-lin e spectrum and the line-ratio variations across the source challenge c onventional power-law photoionization models. In particular, the [N(II )] lambda6584/Halpha ratio is significantly larger than predicted by m odels with solar abundances and, together with [S II] lambdalambda(671 7 + 6731)/Halpha, remains roughly constant across the nebula along PA 105-degrees and PA 15-degrees. In contrast, line ratios involving [O I II] lambda5007 show distinct variations along the direction perpendicu lar to the radio axis (PA 15-degrees), in a pattern consistent with ph otoionization by an anisotropic radiation field emanating from a centr al source. It is likely that the N/O abundance ratio is significantly enhanced (N/O approximately 4 x solar), and that a distributed source of ionization is required in addition to the photoionization by the ac tive nucleus. Consequently, while the ionizing radiation field appears to be anisotropic, the uncertainty as to the ionization mechanism mea ns that we can place only a lower limit on the power of the central co ntinuum source.