The Faint End of the Quasar Luminosity Function at $z \sim 5$ from the Subaru Hyper Suprime-Cam Survey

TL;DR

This study measures the quasar luminosity function at redshift around 5 using Subaru HSC survey data, revealing a flatter faint-end slope and lower number density of low-luminosity quasars than previous studies, with little evolution in certain parameters across redshifts.

Contribution

First measurement of the faint end of the quasar luminosity function at z~5 from wide-field Subaru HSC data, extending previous work to lower luminosities and providing new insights into quasar evolution.

Findings

Fitted a double power-law model with a break magnitude of -25.05 mag.

Found a flatter faint-end slope of -1.22 compared to earlier studies.

Observed a rapid decline in low-luminosity quasar density from z~5 to z~6.

Abstract

We present the quasar luminosity function at $z \sim 5$ derived from the optical wide-field survey data obtained as a part of the Subaru strategic program (SSP) with Hyper Suprime-Cam (HSC). From $\sim$81.8 deg$^2$ area in the Wide layer of the HSC-SSP survey, we selected 224 candidates of low-luminosity quasars at $z \sim 5$ by adopting the Lyman-break method down to $i = 24.1$ mag. Based on our candidates and spectroscopically-confirmed quasars from the Sloan Digital Sky Survey (SDSS), we derived the quasar luminosity function at $z \sim 5$ covering a wide luminosity range of $-28.76 < M_{\rm 1450} < -22.32$ mag. We found that the quasar luminosity function is fitted by a double power-law model with a break magnitude of $M^{*}_{1450} = -25.05^{+0.10}_{-0.24}$ mag. The inferred number density of low-luminosity quasars is lower, and the derived faint-end slope, $-1.22^{+0.03}_{-0.10}$, is flatter than those of previous studies at $z \sim 5$. A compilation of the quasar luminosity function at $4 \leq z \leq 6$ from the HSC-SSP suggests that there is little redshift evolution in the break magnitude and in the faint-end slope within this redshift range, although previous studies suggest that the faint-end slope becomes steeper at higher redshifts. The number density of low-luminosity quasars decreases more rapidly from $z \sim 5$ to $z \sim 6$ than from $z \sim 4$ to $z \sim 5$.