Abstract
For a sample of 20 quasars (3 ≤sssim z ≤sssim 5) we used emission-line ratios to estimate nuclear gas metallicities and the Fe II UV/Mg II line ratio was employed to probe for effects of differential metal enrichment timescales between iron and alpha-elements. The quasars show enhanced solar metallicities (∼5 Z_⊙) in their broad emission-line region and no indication of a metallicity evolution up to redshifts z ≃ 5. The measured Fe II UV/Mg II ratios range from 3 to 5, typical for high redshift quasars, with no indication for evolution with redshift. However, there appears to be a weak tendency for a lower mean Fe II UV/Mg II ratio for z > 4.7. In concert, the gas metallicity, the Fe II UV/Mg II ratio, and model-based estimated time scales for enriching the gas suggest that an intense episode of star formation began at redshifts z = 8 to 13.
