Iron emission in z≈6 QSOs

We have obtained low-resolution near-infrared spectra of three QSOs at 5.7 < z < 6.3 using the Near-Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope. The spectra cover the rest-frame ultraviolet emission of the objects between lambda(rest) approximate to 1600 and 2800 Angstrom. The Fe II emission-line complex at 2500 Angstrom is clearly detected in two of the objects and possibly detected in the third. The strength of this complex and the ratio of its integrated flux to that of Mg II lambda2800 are comparable to values measured for QSOs at lower redshifts and are consistent with Fe/Mg abundance ratios near or above the solar value. There thus appears to be no evolution of QSO metallicity to z approximate to 6. Our results suggest that massive, chemically enriched galaxies formed within 1 Gyr of the big bang. If this chemical enrichment was produced by Type Ia supernovae, then the progenitor stars formed at z approximate to 20 +/- 10, in agreement with recent estimates based on the cosmic microwave background. These results also support models of an evolutionary link between star formation, the growth of supermassive black holes, and nuclear activity.