FeII/Mg II emission-line ratios of QSOs.: II.: z>6 objects

Near-infrared spectra of four QSOs located at z > 6 are obtained with the OH-airglow suppressor mounted on the Subaru telescope. The Fe II/Mg II emission-line ratios of these QSOs are examined by the same fitting algorithm as in our previous study of z < 5.3 QSOs. The fitting results show that two out of the four z > 6 QSOs have significant Fe II emission in their rest-UV spectra, while the other two have almost no Fe II features. We also applied our fitting algorithm to more than 10,000 SDSS QSOs and found two trends in the distribution of Fe II/Mg II against redshift: (1) the upper envelope of the Fe II/Mg II distribution at z > 3 shows a probable declination toward high redshift, and ( 2) the median distribution settles into lower ratios at z similar to 1.5 with small scatter compared to the other redshift. We discuss an Fe/Mg abundance evolution of QSOs with a substantial contribution from the diverse nature of the broad-line regions in high-redshift QSOs.