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Results are presented from a detailed optical/radio study of powerful radio galaxies containing extended emission-line regions (EELRs). Optical spectroscopic datacubes at low and high dispersion have been produced from AAT observations of the EELRs, and multi-waveband radio images of the extended source structure have been obtained with the Australia Telescope. The physical properties of the emission-line gas, including its ionization, excitation and dynamics, are examined in the context of a new model involving ``auto-ionizing'' shocks as the dominant ionization mechanism. The primary aspect of this model involves fast shocks moving through gas clouds colliding at typical relative velocities of $200 - 300$~km/s. Material in the shocked region is heated to a temperature of order $10^6$~K and radiates most of its energy as soft X-rays/UV. This provides a local source of ionizing photons for the surrounding gas, the resulting photoionization spectrum being supplemented by cooling lines from the shocked material.
A comparison is presented between the viability of this model and nuclear photoionization of the EELR, in terms of accounting for the observed line ratios, luminosities and gas dynamics across the entire EELR. It is shown that the ``auto-ionizing'' shock model successfully accounts for the observed line ratios, and in addition is fully self-consistent with the emission line luminosities and velocity differences observed in the gas. Modelling has been carried out using the new MAPPINGS~2 ionization code, as well as SPH simulations of infalling gas in the potential of a large elliptical. A number of possibilities for the origin of shocks in the gas are considered, including interactions between streams of gas during a merger or accretion event, or turbulence resulting from interactions between the radio jets and the gas. This is discussed from the perspective of the origin of the gas in such sources, as well as its possible relation to the large-scale radio properties.
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