1997A&A...323...21V

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 emission line regions (EELR). We find that the UV line ratios cannot be explained in terms of photoionization of solar abundance gas by the classical power law of index α=-1.5, which successfully reproduces the general trends defined by the optical line ratios of low redshift radio galaxies. Pure shock models also provide a poor fit to the data. However, photoionization by a power law of index -1.0 provides an excellent fit to the UV line ratios. This suggests that the ionizing continuum spectral shape may depend on radio luminosity and/or redshift, such that it becomes harder as the radio power and/or redshift increase. However, 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 the power-law photoionization models provide a better fit to the data than the shock models. However, such is the complexity of the shock models that we cannot rule out the possibility that a different combination of input parameters can reproduce the observed spectra. We further show that the UV line ratios provide a sensitive test of the ionization mechanism for the lower ionization conditions prevalent in some low redshift jet-cloud interaction candidates. For high ionization parameter this discrimination is difficult due to the overlap of shock and power law photoionization models.