Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XVII. Black Hole Mass Distribution at z ∼ 6 Estimated via Spectral Comparison with Low-z Quasars

We report the distribution of black hole (BH) masses and Eddingont ratios estimated for a sample of 131 low luminosity quasars in the early cosmic epoch (5.6 < z < 7.0). Our work is based on the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which has constructed a low luminosity quasar sample down to M ₁₄₅₀ ∼ − 21 mag, exploiting the survey data of Hyper Suprime-Cam installed on Subaru Telescope. The discovery spectra of these quasars are limited to the rest-frame wavelengths of ∼1200-1400 Å, which contain no emission lines that can be used as BH mass estimators. In order to overcome this problem, we made use of low-z counterpart spectra from the Sloan Digital Sky Survey, which are spectrally matched to the high-z spectra in overlapping wavelengths. We then combined the C iv emission line widths of the counterparts with the continuum luminosity from the SHELLQs data to estimate BH masses. The resulting BH mass distribution has a range of ∼10^7-10 M _⊙, with most of the quasars having BH masses ∼10⁸ M _⊙ with sub-Eddington accretion. The present study provides not only a new insight into normal quasars in the reionization epoch, but also a new promising way to estimate BH masses of high-z quasars without near-infrared spectroscopy.