Program: GN-2015A-Q-203

Investigating metallicity of QSOs and its redshift evolution is highly important for constraining the main formation epoch of QSO host galaxies, and accordingly probing the origin of the galaxy-SMBH coevolution. Recent observations suggest no clear evidence of the metallicity evolution of QSOs even up to z~6, which is apparently in contradiction to the significant metallicity evolution of massive star-forming galaxies at z~3. One possible scenario to reconcile this apparent contradiction is as following: since we see only exceptionally luminous QSOs at each cosmological epoch due to selection effects, the previous high-z studies were biased to already metal-enriched, "matured" QSOs. Thanks to the SDSS DR9, we have constructed a new sample of high-z QSOs with relatively low luminosity in order to overcome this luminosity bias. We propose to investigate their FeII/MgII flux ratio, that is a well-known "cosmological clock" for the chemical evolution of galaxies. The unique combination of our new QSO sample, the powerful photon-collecting power of the Gemini telescope, and a wide wavelength coverage of GNIRS enables us to identify the chemically young QSOs.