8. Summary and conclusions
We have studied the UV line ratios of a sample of high redshift radio galaxies to understand which is the main mechanism responsible for the line emission processes.
The models which reproduce the optical line ratios of low redshift EELR (-1.5 power law or hot black body and solar abundances of the gas) cannot explain the UV line ratios of very high redshift radio galaxies. We have investigated several possibilities to explain these discrepancies. The most suggestive one, i.e. shocks produced in jet/cloud interactions, fails to explain the data.
On the contrary, a harder ionizing AGN continuum (power law of index =-1) and the classical -1.5 power law with a low metallicity produce an excellent fit to the data. The first possibility is supported by the existing evidence of an evolution towards harder continuum at higher redshifts of the mean spectral shape of the AGN continuum. The second possibility is supported by the evidence provided by studies of absorption line systems in the line of sight of quasars, which demonstrate abundance of 0.1 solar at very high redshifts.
The UV diagnostic diagrams do not distinguish unambiguously between jet-induced shocks and AGN photoionization for the HZRG with high ionization state because the two sets of models overlap in the diagrams. We note, however, that not only are the sequences on the diagnostic diagrams better explained in terms of photoionization, but the overall ionization state of the HZRG is considerably higher than is measured in well-studied jet-cloud interactions at lower redshifts. Our result do not contradict the existence of shocks in these objects. Rather it suggests that the continuum emitted by the central AGN is the main ionizing source, with the shocks having an important effect on the kinematics and morphology of the gas.
For the lower ionization states, the distinction between the shock and photoionization models is more clear-cut, and the UV diagnostic diagrams are a promising means of distinguishing the major ionization mechanism in the low redshift jet-cloud interaction candidates.
Finally, we note that it is always dangerous to base strong conclusions about the ionization mechanisms on measurements of just a few emission lines in a particular spectral range. Our results are suggestive but not conclusive. It is now essential to check the consistency of the model results by obtaining spectra which cover diagnostics in both the optical and UV for individual high and low redshift radio galaxies.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998