Ionisation, shocks and evolution of the emission line gas of distant 3CR radio galaxies

An analysis of the kinematics and ionisation state of the emission line gas of a sample of 14 3CR radio galaxies with redshifts z ∼ 1 is carried out. The data used for these studies, deep long–slit spectroscopic exposures from the William Herschel Telescope, are presented in an accompanying paper. It is found that radio sources with small linear sizes ( ∼ < 150kpc) have lower ionisation states, higher emission line fluxes and broader line widths than larger radio sources. An analysis of the low redshift sample of Baum et al. demonstrates that radio galaxies at low redshift show similar evolution in their velocity structures and emission line ratios from small to large radio sources. The emission line ratios of small radio sources are in agreement with theoretical shock ionisation predictions and their velocity profiles are distorted. Together with the other emission line properties this indicates that shocks associated with the radio source dominate the kinematics and ionisation of the emission line gas during the period that the radio source is expanding through the interstellar medium. Gas clouds are accelerated by the shocks, giving rise to the irregular velocity structures observed, whilst shock compression of emission line gas clouds and the presence of the ionising photons associated with the shocks combine to lower the ionisation state of the emission line gas. By contrast, in larger sources the shock fronts have passed well beyond the emission line regions; the emission line gas of these larger radio sources has much more settled kinematical properties, indicative of rotation, and emission line ratios consistent with the dominant source of ionising photons being the active galactic nucleus. This strong evolution with radio size of the emission line gas properties of powerful radio galaxies mirrors the radio size evolution seen in the nature of the optical– ultraviolet continuum emission of these sources, implying that the continuum alignment effect is likely to be related to the same radio source shocks.