Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). VI. Black Hole Mass Measurements of Six Quasars at 6.1<z<6.7. (arXiv:1904.07278v1 [astro-ph.GA])

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). VI. Black Hole Mass Measurements of Six Quasars at 6.1We present deep near-infrared spectroscopy of six quasars at 6.15.8 quasars with measured black hole masses. From single-epoch mass
measurements based on MgII {lambda}2798, 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 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 build-up 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.

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 MgII {lambda}2798, 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 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 build-up 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.

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