The evidence for jet-cloud interactions in a sample of high-redshift radio galaxies

We present detailed long-slit spectroscopic observations of a sample of four powerful radio galaxies, with high redshifts (0.47 < z < 0.81). The observations cover the rest-wavelength range 2600-6600 Angstrom, chosen to include important diagnostic emission Lines ([Nev]h3426, [O II]lambda 3727, [Ne III]lambda 3869, H beta, [O III]lambda 5007), which are also measured in optical observations of low-redshift radio galaxies. In two of the galaxies (3C 352 and 435A) the radio sources are of the same scale as the emission-line regions, whereas in the other two (3C 34 and 330) the radio sources are extended on a larger scale than the emission-line structures. We find that the extended regions of all the galaxies present highly disturbed kinematics, consisting of line-splitting (Delta upsilon similar to 1000 kms(-1)) and/or underlying broad components (FWHM= 1000-1500 kms(-1)). These features are difficult to explain in terms of gravitational motion in the haloes of the host galaxies. Rather, it is Likely that they are the result of strong shocks driven through the interstellar/intergalactic medium by the radio sources. These observations suggest that jet-induced shocks have an important effect on the emission-line properties even in sources in which the radio structures are on a much larger scale than the emission-line structures. While the emission-line kinematics provide strong evidence for shock acceleration, the dominant ionization mechanism for the emission-line gas remains uncertain. We have compared the optical diagnostic line ratios of the galaxies in our sample with various ionization models, including pure-shock ionization, shocks including a photoionized precursor, power-law photoionization, and photoionization including matter-bounded clouds. We find that both pure-shock ionization and power-law photoionization model predictions fail to provide good fits to the data. On the other hand, on individual diagnostic diagrams, models for shocks which include a photoionized precursor are consistent with the results for the majority of the extended emission-line regions (EELR) of the galaxies in our sample, and photoionization including matter-bounded clouds models also give reasonable fits to some of the EELR. However, in terms of the positions of the points relative to the model sequences on the diagnostic diagrams, there is a lack of consistency from diagram to diagram. The diagnostic diagram involving the line ratios [O III](4959+5007)/4363 and He II(4686)/H beta is particularly problematic in this regard. Overall, our results suggest that, if the EELR are shock-ionized, one or more of the assumptions implicit in the shock models may need to be reconsidered. In addition, we have investigated the nebular continuum contribution to the UV excess in the galaxies in our sample. We find a substantial nebular emission contribution to the UV continuum in all the cases, in the range similar to 10-40 per cent. However, after the subtraction of the nebular component, a significant UV excess remains in the extended nebulae of most of the objects.