(Proposal ID) S12A-003

(PI) Nagao, Tohru

(Proposal Title)
 A Search for Chemically Young QSOs in High-z Universe

(Abstract)
Metallicity of QSOs and its redshift evolution is highly interesting to 
identify the main formation epoch of QSO host galaxies and accordingly to 
explore the origin of the galaxy-SMBH coevolution. Recent observations suggest 
that there are no evidences of the metallicity evolution in QSOs even up to 
z~ 6, that is apparently in contradiction to the significant metallicity of 
massive star-forming galaxies at z~ 3 reported recently. One possible scenario 
to reconcile this apparent contradiction is as following: since we see only 
exceptionally luminous QSOs at each cosmological epoch due to the selection 
effect, we have observed already "matured" QSOs in previous high-z QSO studies.
Thanks to the wide and deep COSMOS survey, we have constructed a new sample of 
high-z lower-luminosity QSOs. 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 for the 
first time.