(Proposal ID) S17A-039

(PI) Onoue, Masafusa

(Proposal Title)
Exploring the Least Massive and Active SMBHs Ever Known at z>6

(Abstract)
Previous studies of z > 6 quasars have suggested that (i) there exist gigantic 
supermassive blackholes (SMBHs) with M_{BH} >10^9 M_{sun} even at z ~ 7 
accreting at the Eddington limit, (ii) the host galaxies are metal-rich, and 
(iii) the SMBHs grow faster than their host galaxies at high redshift. However,
our current knowledge is strongly biased towards the most active quasars 
hosting the most massive BHs. This proposal aims to explore more general SMBHs 
through mass measurements of five low-luminosity quasars at 6.09 < z < 6.80 
with NIR spectroscopy, which are the key population for the scenario of SMBH 
early growth and their relative growth with the host galaxies. Our target 
selection is complete in ~100 deg^2 of the HSC-SSP survey (S15B fields), which 
enables us to derive a complete census of SMBHs at the lowest mass range ever 
(M_{BH}~10^8M_{sun}) at z~ 6. Our targets also include one extremely faint 
quasar at z=6.8 (y_{AB}=23.7), which is likely to host the first ~10^7 M_{sun} 
BH at the highest redshift for quasar studies. Besides the mass measurements, 
we also aim to reveal the chemical composition of those low-luminosity quasars 
using various metal lines and measure the Mg{ii} redshift, which is critical 
for measurement of HII region sizes around the central quasars.