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

We present detailed long-slit spectroscopic observations of a sample of fou r powerful radio galaxies, with high redshifts (0.47 < z < 0.81). The obser vations cover the rest-wavelength range 2600-6600 Angstrom, chosen to inclu de 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 op tical 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-li ne 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 kinematic s, consisting of line-splitting (Delta upsilon similar to 1000 kms(-1)) and /or underlying broad components (FWHM= 1000-1500 kms(-1)). These features a re difficult to explain in terms of gravitational motion in the haloes of t he host galaxies. Rather, it is Likely that they are the result of strong s hocks driven through the interstellar/intergalactic medium by the radio sou rces. These observations suggest that jet-induced shocks have an important effect on the emission-line properties even in sources in which the radio s tructures are on a much larger scale than the emission-line structures. While the emission-line kinematics provide strong evidence for shock accele ration, the dominant ionization mechanism for the emission-line gas remains uncertain. We have compared the optical diagnostic line ratios of the gala xies in our sample with various ionization models, including pure-shock ion ization, shocks including a photoionized precursor, power-law photoionizati on, 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 diagnost ic diagrams, models for shocks which include a photoionized precursor are c onsistent with the results for the majority of the extended emission-line r egions (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 seq uences on the diagnostic diagrams, there is a lack of consistency from diag ram to diagram. The diagnostic diagram involving the line ratios [O III](49 59+5007)/4363 and He II(4686)/H beta is particularly problematic in this re gard. 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 rec onsidered. In addition, we have investigated the nebular continuum contribution to the UV excess in the galaxies in our sample. We find a substantial nebular emi ssion contribution to the UV continuum in all the cases, in the range simil ar to 10-40 per cent. However, after the subtraction of the nebular compone nt, a significant UV excess remains in the extended nebulae of most of the objects.