SIMBAD references

We present observations of 11 high redshift quasars (3.9≲z≲5.0) observed with low spectral resolution in the restframe ultraviolet using FORS1 at the VLT UT1. The emission-line fluxes of strong permitted and intercombination ultraviolet emission lines are measured to estimate the chemical composition of the line emitting gas. Comparisons to photoionization calculations indicate gas metallicities in the broad emission line region in the range of solar to several times solar. The average of the mean metallicity of each high-z quasar in this sample is Z/Z_☉=4.3±0.3. Assuming a chemical evolution time scale of τ_evol≃0.5-0.8Gyrs, we derive a redshift of z_f≃6 to 8 for the onset of the first major star formation episode (H₀=65km/s/Mpc, Ω_M=0.3, Ω_Λ=0.7), corresponding to an age of the universe of several 10⁸yrs at this epoch. We note that this epoch is also supposed to be the era of re-ionization of the universe.