Deep spectroscopy of z~ 1 6C radio galaxies - II. Breaking the redshift-radio power degeneracy

The results of a spectroscopic analysis of 3CR and 6C radio galaxies at redshift z~ 1 are contrasted with the properties of lower-redshift radio galaxies, chosen to be matched in radio luminosity to the 6C sources studied at z~ 1, thus enabling the redshift-radio power degeneracy to be broken. Partial rank correlations and principal component analysis have been used to determine which of redshift and radio power are the critical parameters underlying the observed variation of the ionization state and kinematics of the emission-line gas. [OII]/Hβ is shown to be a useful ionization mechanism diagnostic. Statistical analysis of the data shows that the ionization state of the emission-line gas is strongly correlated with radio power, once the effects of other parameters are removed. No dependence of ionization state on cosmic epoch is observed, implying that the ionization state of the emission-line gas is solely a function of the properties of the active galactic nucleus rather than the host galaxy and/or environment. Statistical analysis of the kinematic properties of the emission-line gas shows that these are strongly correlated independently with both redshift and radio power. The correlation with redshift is the stronger of the two, suggesting that host-galaxy composition or environment may play a role in producing the less extreme gas kinematics observed in the emission-line regions of low-redshift galaxies. For both the ionization and kinematic properties of the galaxies, the independent correlations observed with radio size are stronger than with either radio power or redshift. Radio source age is clearly a determining factor for the kinematics and ionization state of the extended emission-line regions.