In the introductory section the unique character of Cygnus A was noted,
being more than 1.5 orders of magnitude more radio luminous than other
nearby Fanaroff and Riley Class II radio galaxies. Due to the steepness
of the radio luminosity function comparable radio galaxies are only
found at redshifts 1 and beyond. Those extremely powerful distant radio
galaxies are objects of great current interest, and the natural question
arises if Cygnus A can be regarded as a nearby example. With reference
to McCarthy (1993),
many high redshift radio galaxies are characterized
by very strong and extended (tens of kpc) narrow emission line regions,
usually aligned with the radio source axis, blue optical-infrared
colours, and spectacular continuum morphologies, again roughly aligned,
with usually high level of optical (restframe ultraviolet) polarization.
Whereas Cygnus A does have a blue excess this is concentrated in the
nuclear region (central few kpc). Furthermore, neither the continuum
nor the emission line morphologies are in any sense comparable to those
seen in many high redshift radio galaxies, in their relation to the
radio source. In addition, the inferred dust mass of ~ 105 -
106 M
(Djorgovski et al. 1991)
is orders of magnitude below
the values measured for some high redshift radio galaxies (e.g.,
Chini and Krügel
1994,
Ivison 1995).
These facts, but more so the observation that Cygnus A is rather radio over-luminous with respect to its far-infrared, optical and X-ray continuum emission, lead us to believe that Cygnus A is anomalously radio loud, and as such not representative of high redshift radio galaxies. Eales (1996) and Dunlop and Peacock (1995) indeed present evidence that some of the high redshift radio galaxy properties are correlated with the radio luminosity of the objects, rather than being a strong cosmological effect. Whereas we cannot rule out the possibility that some high redshift radio galaxies may also be anomalously radio loud and we eagerly await measurements in this respect, we feel (cf. Eales 1996) that evidence points towards classifying Cygnus A as truly a unique, remarkable source.
Acknowledgement We would like to acknowledge NRAO for providing a pleasant atmosphere in Greenbank during the May 1995 Cygnus A Workshop, and Dan Harris, Dave Clarke, and Clive Tadhunter for commenting upon this work. CLC would like to thank R. Perley and J. Dreher for fostering his involvement in the study of Cygnus A. PDB acknowledges travel support from the Leids Kerkhoven-Bosscha Fund. We would also like to thank the various authors referenced in the figure captions for allowing reproduction of their figures. We made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory at Caltech, under contract with NASA/IPAC.