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 is best fit by models which 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 \pm 120 \mbox { cm } ^ { -3 } and a mean temperature of 11500 \pm 1500 K. Assuming a single population of optically thick line-emitting clouds , we calculate a mean photoionization parameter of ( 1.1 \pm 0.5 ) \times 10 ^ { -2 } and hence a photoionizing photon luminosity of Q \sim 10 ^ { 55 } -10 ^ { 56 } \mbox { phots } ^ { -1 } \mbox { 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-media models , which give Q \sim 5 \times 10 ^ { 53 } -5 \times 10 ^ { 54 } , represent a more likely scenario .

As a by-product of the continuum subtraction we infer that young stellar populations account for \sim 0.4 \% of the visible stellar mass in the galaxy , and that these populations are spatially extended .