Deep spectroscopy of the FUV-optical emission lines from a sample of radio galaxies at z similar to 2.5: metallicity and ionization

We present long-slit near-infrared (NIR) spectra, obtained using the Infrared Spectrometer And Array Camera (ISAAC) instrument at the Very Large Telescope, which sample the rest-frame optical emission lines from nine radio galaxies at z similar to 2.5. One-dimensional spectra have been extracted and, using broad-band photometry, have been cross-calibrated with spectra from the literature to produce line spectra spanning a rest wavelength of similar to 1200-7000 angstrom. The resulting line spectra have a spectral coverage that is unprecedented for radio galaxies at any redshift. We have also produced a composite of the rest-frame ultraviolet (UV)-optical line fluxes of powerful, z similar to 2.5 radio galaxies. We have investigated the relative strengths of Ly alpha, H beta, H alpha, He II lambda 1640 and He II lambda 4687, and we find that A(v) can vary significantly from object to object. In addition, we have identified new line ratios to calculate electron temperature: [NeV] lambda 1575/[NeV] lambda 3426, [NeIV] lambda 1602/[NeIV]lambda 2423, O III] lambda 1663/[O III] lambda 5008 and [O II] lambda 2471/[O II] lambda 3728. We calculate an average O III temperature of 14100(-600)(+1000) K. We have modelled the rich emission line spectra, and we conclude that they are best explained by active galactic nucleus (AGN) photoionization with the ionization parameter U varying between objects. For shock models (with or without the precursor) to provide a satisfactory explanation for the data, an additional source of ionizing photons is required - presumably the ionizing radiation field of the AGN. Single slab photoionization models are unable to reproduce the high- and the low-ionization lines simultaneously: the higher ionization lines imply higher U than do the lower ionization lines. This problem may be alleviated either by combining two or more single slab photoionization models with different U, or by using mixed-medium models such as those of Binette, Wilson & Storchi-Bergmann. In either case, U must vary from object to object. On the basis of N V/N IV] and N IV]/C IV we argue that, while photoionization is the dominant ionization mechanism in the extended emission line regions (EELR), shocks make a fractional contribution (similar to 10 per cent) to its ionization. The N V/N IV] and N IV]/C IV ratios in the broad-line region (BLR) of some quasars suggest that shock ionization may be important in the BLR also. We find that in the EELR of z similar to 2 radio galaxies the N/H abundance ratio is close to its solar value. We conclude that N/H and metallicity do not vary by more than a factor of 2 in our sample. These results are consistent with the idea that the massive ellipticals which become the hosts to powerful AGN are assembled very early in the history of the universe, and then evolve relatively passively up to the present day.