Testing the photoionization models of powerful radio galaxies: Mixed line-emitting media in 3C 321
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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 +/- 120 cm-3 and a mean temperature of 11500 +/- 1500 K. Assuming a single population of optically thick line-emitting clouds, we calculate a mean photoionization parameter of (1.1 +/- 0.5) x 10e-2 and hence a photoionizing photon luminosity of Q ~ 10e55 -- 10e56 photon/s/sr. 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 ~ 5 x 10e53 -- 5 x 10e54, represent a more likely scenario. As a by-product of the continuum subtraction we infer that young stellar populations account for ~ 0.4% of the visible stellar mass in the galaxy, and that these populations are spatially extended. (Refer to PDF file for exact formulas).