The optical emission-line spectrum of Cygnus A

Spectrophotometric measurements are reported of the radio galaxy 3C 405=Cyg A, made using the image-tube image-dissector scanner on the Lick 120-inch (3 m) telescope. The measurements were reduced to energy units by comparison with scans of standard stars made with the same system on the same nights. The emission lines and continuum were measured in the spectral region lambdalambda3346-6731 (in the rest system of Cyg A). The interstellar extinction was determined from the measured Balmer-line ratios, assuming a Case B recombination spectrum and the standard Whitford reddening curve, and the measured line and continuum strengths were corrected for this extinction. The corrected line strengths are discussed for the information they contain on the physical conditions and the energy-input mechanism to the ionized gas. Photoionization by stars is ruled out by the great strength of O i, N i, and S ii. Shock-wave heating is ruled out by the O iii temperature, unless a large amount of ultraviolet ionizing radiation is emitted in the shock. Published calculations of photo-ionization by a synchrotron spectrum, extending far into the ultraviolet, approximately match the observed emission-line spectrum. Likewise, the observed Crab Nebula spectrum approximately matches Cyg A, except for abundance differences, in agreement with the idea that Cyg A is photionized by a power-law spectrum