The structure of the narrow-line region in Cygnus A

We present intermediate-resolution spectra for the powerful radio galaxy Cygnus A taken with the ISIS spectrograph on the William Herschel Telescope. The spectra show complex emission-line profiles across the entire extent of the narrow-line region (NLR) along the radio axis (PA 105°), with line splittings (Δ V ∼ 300 km s - 1 ) at radial distances of 1-2 kpc to both the north-east and south-west, and a combination of broad (500 < FWHM < 800 km s - 1 ) and narrow (FWHM < 300 km s - 1 ) components within a radial distance of 1 kpc of the nucleus. The wide spectral coverage of the data allows us for the first time to measure the reddening, physical conditions and ionization of the kinematic subcomponents individually. Only the broad component detected in the near-nuclear regions shows evidence for significant reddening over and above that due to the Galaxy. The level of reddening of the nuclear broad component is consistent with the idea that the extinction associated with the kiloparsec-scale dust lane induces a mild (≤factor 2) anisotropy in the total emission-line flux, although there is no evidence for a compact, high-density inner narrow-line region in this source. Following correction for reddening, we find a remarkable degree of uniformity in the physical conditions and ionization state between the various spatial and kinematic subcomponents, suggesting a common ionization mechanism for all the subcomponents, despite the differences in kinematics. We use diagnostic diagrams to demonstrate that photoionization by the active galactic nucleus is the dominant ionization mechanism. However, a combination of matter- and radiation-bounded photoionized components is required to explain the strength of the [Fe VIII] λ6087 line and the high electron temperature measured from the [O III] (5007 + 4959)/4363 diagnostic ratio. A major outstanding issue for our understanding of the NLR gas in this and other radio galaxies is how the uniformity in the ionization and physical conditions can be maintained in a mixed photoionized medium across a range of spatial scales and kinematic subcomponents.