Deep spectroscopy of z ∼ 1 6C radio galaxies -: I.: The effects of radio power and size on the properties of the emission-line gas

The results of deep long-slit optical spectroscopy for a sample of eight 6C radio galaxies at redshift z similar to 1 are presented. Emission-line ratios are derived for many emission lines with rest-frame wavelengths of 1500-4500 Angstrom and the kinematic properties of the emission-line gas are derived from an analysis of the two-dimensional structure of the [O II] 3727-Angstrom emission line at approximate to5 Angstrom spectral resolution. In general, the 6C spectra display many characteristics similar to those of more powerful 3CR sources at the same redshifts. The emission-line region gas kinematics are more extreme for the smaller radio sources in the sample, which often display distorted velocity profiles. The ionization state of the emission-line region also varies with radio size: the spectra of large radio sources (>120 kpc) are consistent with photoionization by an obscured active galactic nucleus (AGN), whilst smaller (<120 kpc) sources typically exist in a lower ionization state and have spectra that are better explained by additional ionization due to shocks associated with the expanding radio source. The kinematic and ionization properties of the 6C radio galaxies are clearly linked. As for the 3CR sources, smaller radio sources also typically possess more extensive emission-line regions, with enhanced emission-line luminosities. A high-velocity emission-line gas component is observed in 6C 1019+39, similar to that seen in 3C 265. It is clear that the best interpretation of the spectra of radio sources requires a combination of ionization mechanisms. A simple model is developed, combining AGN photoionization with photoionization from the luminous shock associated with the expanding radio source. The relative contributions of ionizing photons from shocks and the central AGN to an emission-line gas cloud vary with radio source size and the position of the cloud. This model provides a good explanation for both the ionization properties of the emission-line regions and the radio size evolution of the emission-line region extents and luminosities.