THE IONIZATION MECHANISM OF THE EXTENDED GAS IN HIGH-REDSHIFT RADIO GALAXIES - SHOCKS OR AGN PHOTOIONIZATION

We have compared the UV line ratios of a sample of very high redshift radio galaxies (HZRG, z > 1.7) with shock and active galactic nuclei ( AGN) photoionization models, with the goal of determining the balance between jet-induced shocks and AGN illumination in the extended emissi on line regions (EELR). We find that the UV line ratios cannot be expl ained in terms of photoionization of solar abundance gas by the classi cal power law of index alpha=-1.5, which successfully reproduces the g eneral trends defined by the optical line ratios of low redshift radio galaxies. Pure shock models also provide a poor fit to the data. Howe ver, photoionization by a power law of index -1.0 provides an excellen t fit to the UV line ratios. This suggests that the ionizing continuum spectral shape may depend on radio luminosity and/or redshift, such t hat it becomes harder as the radio power and/or redshift increase. How ever, an alternative possibility is that we are seeing the first signs of chemical evolution in these objects, since a power-law of index -1 .5 with low metallicity also provides a very good fit to the data. For the high ionization conditions found in the the HZRG, we show that th e power-law photoionization models provide a better fit to the data th an the shock models. However, such is the complexity of the shock mode ls that we cannot rule out the possibility that a different combinatio n of input parameters can reproduce the observed spectra. We further s how that the UV line ratios provide a sensitive test of the ionization mechanism for the lower ionization conditions prevalent in some low r edshift jet-cloud interaction candidates. For high ionization paramete r this discrimination is difficult due to the overlap of shock and pow er law photoionization models.