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

Abstract

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 $\alpha =-1.23_{-0.34}^{+0.44}$, a bright-end slope $\beta =-2.73_{-0.31}^{+0.23}$, a break magnitude $M_{1450}^{\ast }=-24.90_{-0.90}^{+0.75}$, and a characteristic space density ${\Phi }^{\ast }=10.9_{-6.8}^{+10.0}$ 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 ${\dot{n}}_{\mathrm{ion}}=10^{48.8\pm 0.1}$ 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.

Accepted for publication in ApJ