THE NATURE OF THE EXTENDED BLUE CONTINUUM IN CYGNUS-A

We have used long-slit spectra to derive spatial continuum profiles at UV, optical and near-IR wavelengths in the central region of Cygnus A . The line-free continuum colour distributions derived from these are used to identify the spatial distribution of, and relative contributio n from, the featureless blue continuum (FBC) in each of the principal nuclear subcomponents in this object. The FBC is spatially extended, c overing a region approximately 4-4.5 arcsec in extent along a P.A. of 105-degrees. Beyond this, the emission is predominantly that of the ol d stellar population in the underlying galaxy. In addition, the core r egion is markedly redder than the adjacent NW and SE nuclear subcompon ents, suggesting considerable reddening/extinction. This is strong evi dence in favour of the presence of a nuclear dust lane within the cent ral region of Cygnus A. The fractional contribution of the FBC has bee n estimated as a function of wavelength in each nuclear subregion. A s triking UV excess is seen in our data, the FBC contributing approximat ely 60-90 per cent of the total continuum flux at lambda < 3800 angstr om (the range reflecting the degree of reddening within the intrinsic old stellar population). The continuum shape of the FBC possesses a br eak at approximately 4100 angstrom, exactly as observed in some steep- spectrum radio quasars (and quite different from the spectra of young star clusters). Whilst our observations are most consistent with a sca ttering origin for the FBC, a definitive solution will only be provide d by spectropolarimetric observations in the blue.