  We present deep near-infrared spectroscopy of six quasars at 6.1<z<6.7 with
VLT/X-Shooter and Gemini-N/GNIRS. Our objects, originally discovered through a
wide-field optical survey with the Hyper Suprime-Cam (HSC) Subaru Strategic
Program (HSC-SSP), have the lowest luminosities (-25.5< M1450<-23.1 mag) of the
z>5.8 quasars with measured black hole masses. From single-epoch mass
measurements based on MgII2798, we find a wide range in black hole masses, from
M_BH=10^7.6 to 10^9.3 Msun. The Eddington ratios L_bol/L_Edd range from 0.16 to
1.1, but the majority of the HSC quasars are powered by M_BH=10^9 Msun
supermassive black holes (SMBHs) accreting at sub-Eddington rates. The
Eddington ratio distribution of the HSC quasars is inclined to lower accretion
rates than those of Willott et al. (2010a), who measured the black hole masses
for similarly faint z=6 quasars. This suggests that the global Eddington ratio
distribution is wider than has previously been thought. The presence of
M_BH=10^9 Msun SMBHs at z=6 cannot be explained with constant sub-Eddington
accretion from stellar remnant seed black holes. Therefore, we may be
witnessing the first buildup of the most massive black holes in the first
billion years of the universe, the accretion activity of which is transforming
from active growth to a quiescent phase. Measurements of a larger complete
sample of z>6 low-luminosity quasars, as well as deeper observations with
future facilities will enable us to better understand the early SMBH growth in
the reionization epoch.
