We examine rest-frame ultraviolet spectra of 70 high-redshift quasars (z ≥ 3.5) to study the chemical enrichment history of the gas closely related to the quasars and thereby estimate the epoch of first star formation. The fluxes of several ultraviolet emission lines were investigated within the framework of the most recent photoionization models to estimate the metallicity of the gas associated with the high-redshift quasars. Standard photoionization parameters and the assumption of secondary nitrogen enrichment indicate an average abundance of Z/Z☉ 4-5 in the line-emitting gas. Assuming a timescale of τevol 0.5-0.8 Gyr for the chemical enrichment of the gas, the first major star formation for quasars with z 4 should have started at a redshift of zf 6-8, corresponding to an age of the universe of several times 108 yr (H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.7). We note that this also appears to be the era of reionization of the universe. Finally, there is some evidence for a positive luminosity-metallicity relation in this high-redshift quasar sample.
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