The emission-line regions of high-redshift QSOs

Emission-line data for 13 high-redshift quasars are presented and analyzed with the aid of photoionization models. A comparison of the profiles of different lines suggests that in 11 of the objects the 1909-A C III intercombination line and the 1549-A C IV line have identical profiles, within the observational uncertainties, which are also very similar to those of L-alpha, N V at 1240 A, and possibly O VI at 1035 A. This suggests an approximately constant ionization parameter, U1, for most of the emitting gas. An analysis of the He II 1640-A/L-alpha ratio and other lines suggests that in many cases L-alpha, and in some also C IV at 1549 A, are suppressed by a factor of about 2. It is suggested that internal dust is the cause for this, and its influence on the ionizing radiation field and the line intensities is shown in detail. Best fits to the spectra of four objects are obtained for cases with and without internal dust, and the inclusion of dust is found to remove many difficulties. All the models require fairly high metal abundances, which are deduced mainly from observations of the N IV 1486-A and N III 1750-A intercombination lines. It is found that a higher-U1 low-mass component is still needed to explain the high-excitation lines of O VI and N V, which may argue in favor of disordered gas motion.