UV rest frame spectroscopy of four high redshift (z>2) active galaxies

Abstract

We present UV spectra of a small sample of high redshift active galaxies: the hyperluminous, gravitationally lensed system SMM J02399-0136 (z=2.8) and three powerful radio galaxies (z~2.5). Extended continuum and emission lines are detected in all objects. The three radio galaxies present typical spectra with dominant Lya and weaker CIV1550, HeII1640 and CIII]1909. The spectrum of SMM J02399-0136 is strikingly different, showing relatively weak Lya and HeII and strong NV1240 relative to the C lines. We find that the weakness of HeII can be explained if the intermediate density narrow line region (n~10^{5-6} cm^-3 dominates the emission line spectrum, rather than the more extended low density gas (n<100 cm^-3). The line ratios of MRC2025-218 suggest that this is also the case in this object. MRC2025-218 and SMM J02399-0136 show the largest NV/HeII and NV/CIV values found in high redshift radio galaxies. The failure of solar abundance models to explain these line ratios and studies of metal abundances in high redshift quasars and radio galaxies, suggest that nitrogen is overabundant in the ionized gas of these objects. Alternative possibilities which we discuss include NV emission from the broad line region and differential amplification by a gravitational lens. We report the discovery of a PCygni profile in the CIV line in the spectrum of MRC2025-218. We detect also CII1334.5 in absorption and PCygni profiles for the lines SiIV1393.8,1402.8. We do not detect any purely photospheric stellar lines. The nature of the absorption features is not clear. It could be due to stars or an associated absorption line system.