Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution to Cosmic Reionization at z = 6

We present new measurements of the quasar luminosity function (LF) at \(z \sim 6\), over an unprecedentedly wide range of the rest-frame ultraviolet luminosity \(M_{1450}\) 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 multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The LF was calculated with a complete sample of 110 quasars at \(5.7 \le z \le 6.5\), which includes 48 SHELLQs quasars discovered over 650 deg\(^2\), 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 \(z \sim 6\) quasars with a well-defined selection function constructed to date, and 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 \(\alpha = -1.23^{+0.44}_{-0.34}\), a bright-end slope \(\beta = -2.73^{+0.23}_{-0.31}\), a break magnitude \(M_{1450}^* = -24.90^{+0.75}_{-0.90}\), and a characteristic space density \(\Phi^* = 10.9^{+10.0}_{-6.8}\) Gpc\(^{-3}\) mag\(^{-1}\). Integrating this best-fit model over the range \(-18 < M_{1450} < -30\) mag, quasars emit ionizing photons at the rate of \(\dot{n}_{\rm ion} = 10^{48.8 \pm 0.1}\) 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.