Quasar elemental abundances at high redshifts

We examine rest-frame ultraviolet spectra of 70 high-redshift quasars (z greater than or equal to 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(circle dot) similar or equal to 4-5 in the line-emitting gas. Assuming a timescale of tau(evol) similar or equal to 0.5-0.8 Gyr for the chemical enrichment of the gas, the first major star formation for quasars with z similar or equal to 4 should have started at a redshift of z(f) similar or equal to 6-8, corresponding to an age of the universe of several times 10(8) yr (H-0 = 65 km s(-1) Mpc(-1), Omega(M) = 0.3, Omega(Lambda) = 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.