Quasar Luminosity Function at z = 7

Abstract: We present the quasar luminosity function (LF) at $z = 7$, measured with 35 spectroscopically confirmed quasars at $6.55 < z < 7.15$. The sample of 22 quasars from the Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs) project, combined with 13 brighter quasars in the literature, covers an unprecedentedly wide range of rest-frame ultraviolet magnitudes over $-28 < M_{1450} < -23$. We found that the binned LF flattens significantly toward the faint end populated by the SHELLQs quasars. A maximum likelihood fit to a double power-law model has a break magnitude $M^*_{1450} = -25.60^{+0.40}_{-0.30}$, a characteristic density $\Phi^* = 1.35^{+0.47}_{-0.30}$ Gpc$^{-3}$ mag$^{-1}$, and a bright-end slope $\beta = -3.34^{+0.49}_{-0.57}$, when the faint-end slope is fixed to $\alpha = -1.2$ as observed at $z \le 6$. The overall LF shape remains remarkably similar from $z = 4$ to $7$, while the amplitude decreases substantially toward higher redshifts, with a clear indication of an accelerating decline at $z \ge 6$. The estimated ionizing photon density, $10^{48.2 \pm 0.1}$ s$^{-1}$ Mpc$^{-3}$, is less than 1 % of the critical rate to keep the intergalactic medium ionized at $z = 7$, and thus indicates that quasars are not a major contributor to cosmic reionization.