DYNAMICS AND EXCITATION OF RADIO GALAXY EMISSION-LINE REGIONS - I - PKS-2356-61

Results are presented from a program of detailed long-slit spectroscop ic observations of the extended emission-line region (EELR) associated with the powerful radio galaxy PKS 2356-61. The observations have bee n used to construct spectroscopic data cubes, which yield detailed inf ormation on the spatial variations of emission-line ratios across the EELR, together with its kinematic structure. We present an extensive c omparison between the data and results obtained from the MAPPINGS II s hock ionization code, and show that the physical properties of the lin e-emitting gas, including its ionization, excitation, dynamics and ove rall energy budget, are entirely consistent with a scenario involving autoionizing shocks as the dominant ionization mechanism. This has the advantage of accounting for the observed EELR properties by means of a single physical process, thereby requiring fewer free parameters tha n the alternative scheme involving photoionization by radiation from t he active nucleus. Finally, possible mechanisms of shock formation are considered in the context of the dynamics and origin of the gas, spec ifically scenarios involving infall or accretion of gas during an inte raction between the host radio galaxy and a companion galaxy.