2018ApJ...869..150M

We present new measurements of the quasar luminosity function (LF) at z ∼ 6 over an unprecedentedly wide range of the rest-frame ultraviolet luminosity M₁₄₅₀ from -30 to -22 mag. This is the fifth in a series of publications from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multiband imaging data produced by the Hyper Suprime-Cam Subaru Strategic Program survey. The LF was calculated with a complete sample of 110 quasars at 5.7 <= z <= 6.5, which includes 48 SHELLQs quasars discovered over 650 deg² and 63 brighter quasars discovered by the Sloan Digital Sky Survey and the Canada-France-Hawaii Quasar Survey (including one overlapping object). This is the largest sample of z ∼ 6 quasars with a well-defined selection function constructed to date, which has allowed us to detect significant flattening of the LF at its faint end. A double power-law function fit to the sample yields a faint-end slope α=-1.23_–0.34^+0.44, a bright-end slope β=-2.73_–0.31^+0.23, a break magnitude M₁₄₅₀^* =-24.90_–0.90^+0.75, and a characteristic space density Φ^* =10.9_–6.8^+10.0 Gpc^–3 mag^–1. Integrating this best-fit model over the range -18 < M₁₄₅₀ < -30 mag, quasars emit ionizing photons at the rate of {dot}n_ion=10^48.8±0.1^ s^–1 Mpc^–3 at z = 6.0. This is less than 10% of the critical rate necessary to keep the intergalactic medium ionized, which indicates that quasars are not a major contributor to cosmic reionization.