Exploring Reionization-Era Quasars III: Discovery of 16 Quasars at $6.4\lesssim z \lesssim 6.9$ with DESI Legacy Imaging Surveys and UKIRT Hemisphere Survey and Quasar Luminosity Function at $z\sim6.7$

TL;DR

This study reports the discovery of 16 quasars at redshifts 6.4 to 6.9, analyzes their luminosity function, and concludes that high-redshift quasars are unlikely to significantly contribute to hydrogen reionization, highlighting rapid SMBH growth in early universe.

Contribution

First to combine DESI, UKIRT, and WISE data for efficient selection of reionization-era quasars, and to measure the quasar luminosity function at z~6.7 with a large sample.

Findings

Discovered 16 quasars at 6.4 < z < 6.9 over 13,020 deg².

Quasar luminosity function follows a power law with index -2.35.

Quasar density declines rapidly with redshift, by a factor of ~6 per unit z.

Abstract

This is the third paper in a series aims at finding reionzation-era quasars with the combination of DESI Legacy imaging Surveys (DELS) and near-infrared imaging surveys, such as the UKIRT Hemisphere Survey (UHS), as well as the Wide-field Infrared Survey Explore ($WISE$) mid-infrared survey. In this paper, we describe the updated quasar candidate selection procedure, report the discovery of 16 quasars at $6.4\lesssim z \lesssim6.9$ from area of $\sim$13,020 deg$^2$, and present the quasar luminosity function (QLF) at $z\sim6.7$. The measured QLF follows $\Phi(L_{1450})\propto L_{1450}^{-2.35}$ in the magnitude range $27.6<M_{1450}<-25.5$. We determine the quasar comoving spatial density at $\langle z \rangle$=6.7 and $M_{1450}<-26.0$ to be $\rm 0.39\pm0.11 Gpc^{-3}$ and find that the exponential density evolution parameter to be $k=-0.78\pm0.18$ from $z\sim6$ to $z\sim6.7$, corresponding to a rapid decline by a factor of $\sim 6$ per unit redshift towards earlier epoch, a rate significantly faster than that at $z\sim 3- 5$. The cosmic time between $z\sim6$ and $z\sim6.7$ is only 121 Myrs. The quasar density declined by a factor of more than three within such short time requires that SMBHs must grow rapidly or they are less radiatively efficient at higher redshifts. We measured quasar comoving emissivity at $z\sim6.7$ which indicate that high redshift quasars are highly unlikely to make a significant contribution to hydrogen reionization. The broad absorption line (BAL) quasar fraction at $z\gtrsim6.5$ is measured to be $\gtrsim$22%. In addition, we also report the discovery of additional five quasars at $z\sim6$ in the appendix.