Program: GN-2017B-Q-53

Investigating the metallicity of QSOs and its redshift evolution is highly important for constraining the main formation epoch of QSO host galaxies. To understand the metallicity evolution, we investigated FeII/MgII ratio, that is a well-known ``cosmological clock'', of high redshift and low luminous QSOs. We detected low FeII/MgII ratio from the low-luminosity QSOs compared to high-luminosity QSOs (Maiolino et al. 2003) suggesting that low-luminosity QSOs are the candidate of chemically young QSOs. Unfortunately, the low quality data with very low resolution (R~75) for high-luminosity QSOs in previous work prevent us to clearly confirm that the low-luminosity QSOs have really low metallicity. Thus, we propose to investigate the FeII/MgII ratio as a function of luminosity to confirm whether low-luminosity QSOs are really chemically young QSOs.