Previous studies of luminous z>6 quasars revealed the existence of billion-solar-mass black holes (BHs) within the first billion years of the universe. Dynamical mass measurements of their host galaxies have suggested that these BHs grow faster than their host galaxies with respect to the local scaling relation. However, those studies are highly biased towards the most massive BHs rapidly accreting at high redshift. We propose nuMOIRCS Ks-band spectroscopy of six low-luminosity quasars at 6.6≤ z≤ 6.9 to measure their virial BH masses based on Mgii λ2798 emission lines. Our targets from the HSC-SSP (yAB∼21–22 mag) are 1–2 magnitudes fainter than those in previous z∼7 studies; thus, our sample is less affected by the luminosity bias. Combining with other HSC-SSP z∼7 quasars with BH masses available, we will characterize the BH mass and Eddington ratio distributions at the highest redshift for the first time. As the total sample size is comparable to similar studies at z∼6, this program enables us to address whether the BH activities are enhanced toward the highest redshift, which has been discussed based on luminosity function. Coordinated ALMA [Cii] 158µm observations for the same targets will address if the SMBH-galaxy co-evolution holds at z∼7.
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