| dc.description.abstract | We present detailed long-slit spectroscopic observations of a sample of four powerful
radio galaxies, with intermediate/high redshifts (0.47 < z < 0.81). The observations cover the rest-wavelength range 2600 °A- 6600 °A, chosen to include important diagnostic emission lines ([NeV] 3426, [OII] 3727, [NeIII] 3869, H , [OIII] 5007), which are also measured in optical observations of low-redshift radio galaxies. In two of the galaxies (3C 352 and 3C 435A) the radio sources are of the same scale as the
emission-line regions, whereas in the other two (3C 34 and 3C 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 ( v 1000 km s−1) and/or underlying broad components (FWHM = 1000 — 1500 km s−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 ISM/IGM 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 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 [OIII](4959+5007)/4363 and HeII(4686)/H 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 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. (Refer to PDF file for exact formulas). | en_US |
