PKS 2250-41 and the role of jet-cloud interactions in powerful radio galaxies

We present high-resolution, long-slit spectra of the jet-cloud interaction in the powerful southern radio galaxy PKS 2250-41. We have resolved the emi ssion lines into two main kinematic components: a broad component (FWHM gre ater than or equal to 900 km s(-1)) and a narrow component (FWHM less than or equal to 150 km s(-1)). While the broad component is characterized by a low ionization level (with particularly weak He II lambda 4686 emission) an d is spatially associated with the radio lobe, the narrow component is char acterized by a higher ionization level and extends well beyond the radio lo be. Crucially, we measure a higher electron temperature for the broad compo nent (T similar to 30000 K) than for the narrow component (T similar to 15 000 K). The general line ratios and physical conditions of the two componen ts are consistent with a model in which the broad component represents gas cooling behind the shock front driven by the radio jets, while the narrow c omponent represents the AGN- or shock-photoionized precursor gas. However, uncertainties remain about the gas acceleration mechanism behind the shock front: unless the radio components are expanding unusually fast in this sou rce, it is likely that entrainment of the warm clouds in the hot post-shock wind or radio plasma is required in addition to the initial acceleration a cross the shock front, in order to explain the large linewidths of the broa d component. The similarities between the kinematic properties of PKS 2250-41 and some h igh-redshift radio galaxies suggest that the ambient and the shocked gas ha ve also been resolved in the more distant objects. Given the evidence that the emission-line processes are affected by the interactions between the ra dio and the optical structures, care must be taken when interpreting the UV spectra of high-redshift radio galaxies.