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

Abstract

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 WHT, are presented in an accompanying paper. It is found that radio sources with small linear sizes (<150 kpc) have lower ionisation states, higher emission line fluxes and broader line widths than larger 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 ISM. Gas clouds are accelerated by the shocks, giving rise to the irregular velocity structures observed, whilst shock compression of the 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; their emission line gas 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. (Abridged)