Detection of Iron Emission in the z = 5.74 QSO SDSSp J104433.04$-$012502.2

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Abstract
We obtained near-infrared spectroscopy of the |$z = 5.74$| QSO, SDSSp J104433.04|$-$|012502.2, with the Infrared Camera and Spectrograph of the Subaru Telescope. The redshift of 5.74 corresponds to a cosmological age of 1.0 Gyr for the current |$\Lambda$|-dominated cosmology. We found a similar strength of the Fe ii (3000–3500 Å) emission lines in SDSSp J104433.04|$-$|012502.2 as in low-redshift QSOs. This is the highest redshift detection of iron. We subtracted a power-law continuum from the spectrum and fitted model Fe ii emission and the Balmer continuum. The rest equivalent width of Fe ii (3000–3500 Å) is |$\sim 30 \,$| Å, which is similar to those of low-redshift QSOs measured in the same manner. The chemical-enrichment models that assume the lifetime of the progenitor of SNe Ia is longer than 1 Gyr predict that weaker Fe ii emission in high-redshift (⁠|$z \gt 3$|⁠) QSOs than low-redshift ones. However, none of the observed high redshift QSOs show a systematic decrease of Fe ii emission compared with low-redshift QSOs. This may be due to a shorter lifetime of SNe Ia in QSO nuclei than in the solar neighborhood. Another reason for the strong Fe ii emission at |$z = 5.74$| may be a longer cosmological age due to a smaller |$\Omega _\mathrm{M}$|⁠.
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