Testing the photoionization models of powerful radio galaxies: mixed line-emitting media in 3C 321

The photoionization models for the narrow emission-line regions of powerful radio galaxies have yet to be tested in depth. To this end, we present high-quality long-slit spectroscopy of the powerful double-nucleus radio galaxy 3C 321. The data have good enough spatial resolution to be able to trace the variation in emission-line properties on kpc scales. Continuum modelling and subtraction enables the faint emission-line fluxes to be measured in several regions across the emission-line nebula. We plot diagnostic line-ratio diagrams and compare them with the predictions of various photoionization models, finding that the data are best fitted by models that assume a mixture of optically thin and thick clouds illuminated by a power-law continuum. The emission-line kinematics, line ratios and deduced physical conditions show remarkably little variation across the source. We deduce a mean electron density of 400 +/- 120 cm(-3) and a mean temperature of 11 500 +/- 1500 K. Assuming a single population of optically thick line-emitting clouds, we calculate a mean photoionization parameter of (1.1 +/- 0.5) x 10(-2), and hence a photoionizing photon luminosity of Q similar to 10(55)-10(56) photon s(-1) sr(-1). This indicates a central engine as luminous as that of the powerful quasar 3C 273, yet there is no evidence for such an energetically prolific central engine at either far-infrared or radio wavelengths. We therefore conclude that the mixed-medium models, which give Q similar to 5 x 10(53)-5 x 10(54) photon s(-1) sr(-1) represent a more likely scenario. As a by-product of the continuum subtraction, we infer that young stellar populations account for similar to 0.4 per cent of the visible stellar mass in the galaxy, and that these populations are spatially extended.