FeII/MgII Emission Line Ratios of QSOs. II. z>6 Objects
Abstract
Near-infrared spectra of four QSOs located at z>6 are obtained with the OH-airglow suppressor mounted on the Subaru telescope. The FeII/MgII 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 FeII emission in their rest-UV spectra, while the other two have almost no FeII features. We also applied our fitting algorithm to more than 10,000 SDSS QSOs and found two trends in the distribution of FeII/MgII against redshift: (1) the upper envelope of the FeII/MgII distribution at z>3 shows a probable declination toward high redshift, and (2) the median distribution settles into lower ratios at z 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.
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