Quasar Luminosity Function at Z=7

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(-0.30)(+ 0.40), a characteristic density Phi* = -1.35(-0.30)(+0.47) Gpc(-3) mag(-1), and a bright end slope beta = -3.34(-0.57)(+0.49), when the faint-end slope is fixed to alpha = -1.2 as observed at z <= 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 >= 6. The estimated ionizing photon density, 10(48.2 +/- 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.