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 hig h-quality long-slit spectroscopy of the powerful double-nucleus radio galax y 3C 321. The data have good enough spatial resolution to be able to trace the variation in emission-line properties on kpc scales. Continuum modellin g and subtraction enables the faint emission-line fluxes to be measured in several regions across the emission-line nebula. We plot diagnostic line-ra tio diagrams and compare them with the predictions of various photoionizati on models, finding that the data are best fitted by models that assume a mi xture of optically thin and thick clouds illuminated by a power-law continu um. The emission-line kinematics, line ratios and deduced physical conditions s how remarkably little variation across the source. We deduce a mean electro n 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 h ence 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 p owerful quasar 3C 273, yet there is no evidence for such an energetically p rolific central engine at either far-infrared or radio wavelengths. We ther efore 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 p opulations account for similar to 0.4 per cent of the visible stellar mass in the galaxy, and that these populations are spatially extended.