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 $M_{1450}^{\ast }=-25.60_{-0.30}^{+0.40}$, a characteristic density $\hat{I}{¦}^{\ast }=1.35_{-0.30}^{+0.47}$ Gpc${}^{-3}$ mag${}^{-1}$, and a bright-end slope ${\hat{I}}^2=-3.34_{-0.57}^{+0.49}$, when the faint-end slope is fixed to $\hat{I}\pm =-1.2$ 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.

The Astrophysical Journal Letters, in press