Kavli Affiliate: Masahiro Takada
| First 5 Authors: Yoshiki Matsuoka, Masafusa Onoue, Kazushi Iwasawa, Michael A. Strauss, Nobunari Kashikawa
| Summary:
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.
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