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4. IONIZATION OF THE GAS IN RADIO GALAXIES

The question of the nature of gas associated with radio jets is intimately connected with the broader issue of the excitation and ionization of emission-line gas in powerful radio galaxies. It seems worthwhile at this point to summarize current knowledge on this more general question. The ionization source for the extended emission-line gas in powerful radio galaxies could be: (i) shock waves powered by the jets or lobes, (ii) local photoionizing radiation powered by the jets or lobes, (iii) relativistic particles associated with the jets or lobes, (iv) nuclear photoionizing radiation, or (v) photoionizing radiation from hot stars, the formation of which was triggered by the radio jet. While clear choices between these processes are often hard to make in individual cases, a number of lines of argument suggest that the gas is mostly photoionized, as I now discuss.