This is a resubmission of an approved proposal which was lost due to the volcanic eruption. High redshift quasars are vital tools for studying the formation and evolution of supermassive black holes (SMBHs) in the early Universe (z>5). Quasars that are particularly bright at radio wavelengths can be uniquely used to chart the evolutionary histories of the SMBHs due to the long timescales needed for bright radio synchrotron emission. A recent campaign using LOFAR data combined with deep archival optical and near-infrared data has discovered 4 radio-loud quasars at z>6 using Subaru/FOCAS and other facilities. We now propose to use Subaru/MOIRCS for one night to obtain deep near-infrared spectra for these 4 quasars, targeting rest-frame UV emission to measure their SMBH masses and Eddington ratios. Combined with radio flux and spectral index measurements already taken with LOFAR and GMRT, the SMBH masses and activity timescales can be uniquely used to accurately constrain the formation epoch of the first SMBH seeds in the Universe.
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