The relations between radio and forbidden [O III]-emission line luminosities in FRII radiogalaxies

We present spectrophotometry of 17 FRII radiogalaxies and use the resulting [O III]4959 + 5007 line fluxes to investigate the relations between radio and [O III] emission-line luminosities in an unbiased sample of 39 FRIIs with z < 0.5. For such objects this work provides the first proof of an empirical link between 178-MHz total luminosity P178 and [O III]4959/5007 line luminosity P[O III]; it also confirms a previously reported link of P178 with the 5-GHz luminosity Prad central of the radio central component. We argue that an apparent correlation between P[O III] and Prad central can be explained by separate P178–P[O III] and P178–Prad central links and need not be intrinsic. The relative strengths of these correlations provide evidence, independent of arguments based on the intensity ratios of the narrow lines, that the narrow-line region is photo-ionized from a central source which is physically distinct from the radio-jets.