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Astron. Astrophys. 323, 21-30 (1997)

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1. Introduction

The character of the emission line spectra of powerful radio galaxies is strongly determined by the ionization mechanism of the gas. For most low redshift ([FORMULA] 0.1) radio galaxies photoionization by a central AGN describes rather well the general properties of the optical emission line spectra (e.g. Robinson et al. 1987). However, when the first high redshift radio galaxies (HZRG, [FORMULA]) were discovered the situation was not so clear. These objects showed very blue colours and strong Ly [FORMULA] emission - properties expected for galaxies in the process of formation (e.g. Spinrad et al. 1985, McCarthy et al. 1987). It was proposed that some of these HZRG, like 3C326.1, were very young galaxies in which the strong Ly [FORMULA] emission was powered by the young blue stars (McCarthy et al. 1987).

However, detailed analysis of the spectra showed that young stars could not explain the general properties of the UV emission lines of HZRG; in particular, the large emission line equivalent widths and the existence of highly ionized species, like CIV and HeII (e.g. McCarthy et al. 1990).

In the context of the unified schemes for powerful radio galaxies (e.g. Barthel 1989), we would expect these galaxies to harbour powerful AGN. This, together with the fact that AGN photoionization succeeded in explaining the optical emission line ratios of low redshift radio galaxies, suggested that the same mechanism dominates the ionization processes in HZRG (McCarthy 1993). In consequence, most attempts to explain the UV emission line spectra of HZRG have invoked pure AGN photoionization.

However, both imaging and spectroscopy of HZRG show that strong interactions are taking place between the advancing radio jet and the ISM of the host galaxy (Sect. 2). Such interactions will generate powerful shocks which will disturb the morphology, kinematics and physical conditions (density, temperature, pressure) of the gas and potentially modify its ionization state. Currently a key issue in the study of these objects is the relative importance of jet-induced shocks and AGN illumination: do shocks dominate the emission line processes? Or is AGN photoionization dominant with the influence of the shocks mainly manifested in kinematic and morphological disturbances?

We have addressed this problem by studying the UV emission line spectrum of a sample of very HZRG ([FORMULA] 1.7), comparing their line ratios with both shock and AGN photoionization models. We base our study on the UV lines because most of the information we have about HZRG is derived from studies of the UV emission line spectra. At such high redshifts the main optical diagnostic emission lines are shifted into the infrared, and most of the existing IR spectra for the HZRG are of poor quality.

In contrast, at low redshifts we have the opposite problem: the optical diagnostic line ratios are well measured, but the UV spectra are of low quality. For the low redshift objects the optical line ratios have proved inefficient at distinguishing the ionization mechanism unambiguously, although it has been suggested that the UV line ratios might provide a stronger discriminant (Sutherland et al. 1993). Therefore, the interest in understanding the emission of the UV lines can be extended to low redshift objects. We propose to develop a diagnostic method to discriminate between shock and AGN ionization in radio galaxies at all redshifts.

We review in Sect. 2 the observational evidence for AGN illumination and shocks in powerful radio galaxies. The data sample is described in Sect. 3 and the diagnostic diagrams in Sect. 4. Sects. 5 and 6 present the models and the comparison with the data: Sect. 5 concentrates on the effects of a) varying the shape of the AGN continuum and b) changing the excitation and ionization mechanism (shocks); Sect. 6 analyzes the effects that different physical conditions in the extended gas can produce in the observed UV spectra. In Sect. 7 we extend our diagnostic method to low redshift radio galaxies. Sect. 8 includes summary and conclusions.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998