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9.2. Optical Emission Components

Whereas Baade and Minkowsky (1954) did notice that forbidden-line emission was strong, it took two decades until Van den Bergh (1976) investigated the Cygnus A galaxy in pure continuum light, and discovered that the double nature of the galaxy center and its resemblance to the Centaurus A galaxy NGC 5128 was mainly due to excess line and continuum emission in the 2 arcsec double morphology. The NW component of the double morphology stood out particularly with strong permitted and forbidden line emission. Subsequent astrometry located the radio core source in between this double optical feature (Kronberg et al. 1977). Later, high resolution optical imaging led to the discovery of a third compact optical emission feature coincident with the radio core (Thompson 1984). We will come back to this issue below, in the section on the nuclear regions.

Schmidt (1965) ranked Cygnus A as an extremely luminous line emitter in a study of powerful radio galaxies. The first extensive study of the emission line spectrum was carried out by Osterbrock and Miller (1975). This seminal spectrophotometric study of the central regions of the Cygnus A galaxy (aperture size approx 3") led to several important discoveries. First, the extinction as inferred from the emission line Balmer decrement was considerably greater than the extinction obtained from broad band aperture photometry (Sandage 1972). This was confirmed by Van den Bergh (1976) and Yee and Oke (1978). The Galactic foreground component was determined using field ellipticals (Van den Bergh 1976, Spinrad and Stauffer 1982). Osterbrock and Miller (1975) explained the measured extinction behavior in terms of additional extinction towards the central emission line regions in Cygnus A (later confirmed by Yee and Oke 1978). An extinction corrected optical continuum slope of -1.6 was determined, which is substantially harder than the value determined using the old reddening correction (Oke 1968). It is important to keep in mind that these data reflect the central regions of the Cygnus A galaxy. Second, the mass of the ionized gas in Cygnus A was determined to be about 107 Msun, the inferred filling factor was low, and the abundances were normal. Thirdly, the best candidate ionizing source appeared to be a non-thermal source of radiation, although a contribution of shock heating could not be ruled out. The extrapolation of the observed continuum (L propto nu-1.6) fell short of producing the observed ionization, by 30%. This is a very important point, to which we will come back below.

Cygnus A became the prototype narrow-line radio galaxy (NLRG); see e.g., Osterbrock (1989). In contrast to other NLRGs however, stellar absorption features such as the calcium H and K lines were not seen. Yee and Oke (1978) had already established that a substantial component of nonthermal continuum contributed to the optical spectrum of Cygnus A, thereby diluting the stellar light. Osterbrock (1983) made the first attempt to quantify the relative contribution of this blue nonthermal continuum, also sometimes described as blue featureless continuum (BFC). He estimated the elliptical galaxy to contribute about 40% and the BFC to contribute about 60% around 5000 Å in a approx 3" aperture. In summary, in the mid-eighties it was clear that a complex interplay was present in the Cygnus A galaxy, consisting of early type stellar continuum, extended narrow emission line gas, blue featureless continuum and dust absorption/reddening.

Pierce and Stockton (1986) obtained line and continuum images as well as long slit spectroscopy in order to assess the relative contributions of the various optical components and to determine their morphology and origin. Their images confirmed and extended Van den Bergh's (1976) results, mapping out the morphology and ionization structure of the extended line emission at 1 kpc resolution. The emission line gas was found to have a clumped and filamentary structure. Just east of the NW emission line cloud, a fainter component coincident with the radio core was found. From its diluting effect on the stellar absorption features, the blue featureless continuum was found to extend over several arcsec in the central region, and ascribed to either a single obscured nucleus (via scattering) or many distributed photoionizing sources. The obscured nucleus model is particularly interesting in light of the proposal that powerful radio galaxies may harbor quasars in their nuclei, obscured by anisotropically distributed dust (Scheuer 1987, Peacock 1987, Barthel 1989), and recalls the Osterbrock and Miller (1975) finding that the extrapolated ultraviolet radiation fell short of producing the observed emission line radiation. Spatial reddening structure was measured, both in the line and in the continuum radiation. The presence of line-emitting filaments (Halpha+[NII]), observed to stretch south and north-east for 5-10 arcsec was confirmed by Carilli et al. (1989). These authors compared emission line gas pressure values with radio synchrotron minimum energy pressures and found the gas to be overpressured except in the northern filament which seems to lie along the trailing edge of the eastern radio lobe. The same authors also determined the galaxy's absolute magnitude to be MR = -24, confirming earlier claims that the object is very luminous. Typical line luminosity values are ~ 1042 erg sec-1 (Carilli et al. 1989, Baum et al. 1989a). These values seem large, but Baum et al. (1989b) show that Cygnus A is somewhat underluminous for its radio luminosity. The [OIII] image obtained by Baum et al. (1988) shows the doubly ionized oxygen gas to be extended over ~ 5 arcsec. This compares to ~ 30 arcsec for the [OII] as inferred from the long-slit spectroscopy of Baade and Minkowsky (1954)! Confirmation of the extent of the [OII] nebula is however needed.

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