CONTINUUM AND LINE EMISSION IN CYGNUS A

We present the results from (1) imaging observations of Cygnus A in fi ve essentially line-free continuum bands with central wavelengths rang ing from 0.34 to 2.1 mum, (2) imaging observations in five narrowband filters centered on the emission lines Hbeta, [O III] lambda5007, Halp ha, [N II] lambda6583, and [S II] lambdalambda6716,6731, and (3) deep spectroscopy covering the entire central region of Cyg A. We confirm t hat the featureless spectrum component is to be identified with the pr ominent double morphology at the center of Cyg A, but uncertainties in the distribution of the dust in this region tend to limit the accurac y with which we can determine its morphology and spectral-energy distr ibution (SED). From regions that appear to be least affected by obscur ation, we find f(v) approximately v-0.1 for this component. This SED c ould be consistent with free-free emission, a population of young star s, or a quasar continuum scattered by electrons, but probably not with a quasar continuum scattered by dust, which would be bluer. Our spect roscopy places an upper limit on the equivalent width of broad Hbeta t hat is well below that of typical quasars, showing that this flat-spec trum component (FSC) is almost certainly not dominated by scattered qu asar radiation. Appeals to scattering by hot electrons to smear the sc attered broad lines into invisibility appear to fail because the large density scale height of the electrons and the inefficiency of electro n scattering should result in smoother and more extensive structure th an we observe. Although the relative SED is consistent with free-free emission, the required amount of hot gas violates other observational constraints' At high angular resolution, the apparent morphology of th e FSC is spiral-like. Although this impression may be partly due to ob scuration, the distribution of the dust itself only serves to reinforc e the spiral-like nature of the material with which it is associated. We conclude that the FSC is most likely radiation from a starburst at the center of Cyg A. Fits to the radial surface-brightness profile of the late-type component after our best efforts at removing the FSC all ow us to estimate the wavelength dependence of extinction in the centr al region. A comparison with simple models for sources embedded in dus t indicates that approximately 50% of the emergent blue flux from the late-type stars at the center may be due to scattered radiation. This large scattered contribution (which probably applies as well to the FS C and the emission-line component), together with a modest asymmetry i n the radiation field, could plausibly account for the observed polari zation properties of the inner region of Cyg A. Systematic differences are present between the distribution of [O III] on the one hand and [ N II] (and other low-ionization species) on the other. After a large c ommon component to the distribution is removed, the remaining [O III] excess is aligned with the radio axis, while the [N II] excess is dist ributed in what appears to be a planar structure perpendicular to the radio axis. The association of dust with this latter component indicat es that it is a distinct morphological structure and not simply the re sult of shadowing effects. The emission-line intensities in the high-i onization regions are consistent with photoionization by a power-law c ontinuum; those in the low-ionization region are similar to those seen in LINERs and appear to be consistent with ratios expected from x-ray photoionization. We find a systemic redshift for Cyg A of 0.05562+/-0 .00015, from a measurement of the strongly diluted Mg I b feature in h igh-S/N spectra of the central region. The velocity field of the ioniz ed gas is quite complex, showing at least two components along the lin e of sight in many regions. In general, discrete clouds show velocity curves consistent with rotation rather than with outflow, but high-vel ocity gas, probably associate with radio jets, is seen on both sides o f the galaxy nucleus. The ionized gas centered on the nucleus is count er-rotating relative to the bright cloud to the NW and most of the fai nter material in the same area. We conclude that, while there are stro ng reasons for believing that Cyg A harbors a hidden quasar (the prese nce of a point x-ray source, evidence from the emission lines of a fla t photoionizing continuum, and the probable recent detection of scatte red broad Mg II), there is no evidence that a significant amount of qu asar optical or IR continuum has been scattered into our line of sight . Although the optical continuum peaks are roughly aligned with the ra dio structure, it is unlikely that there is any physical correspondenc e with the alignment between continuum and radio structure seen in lum inous radio galaxies at high redshifts. Finally, the counter-rotating gas in the nuclear region, the secondary IR peak 1'' N of the nucleus, the evidence for star formation in the nuclear mini-spiral, and the e xtensive ragged swath of low-ionization gas and dust stretching to the NE and S all suggest that Cyg A has suffered a recent merger.