2019ApJ...884...30W

This is the third paper in a series aimed at finding reionization-era quasars with the combination of DESI Legacy imaging Surveys (DELS), the Pan-STARRS1 (PS1) Survey, and near-infrared imaging surveys, such as the UKIRT Hemisphere Survey (UHS), as well as the Wide-field Infrared Survey Explorer (WISE) mid-infrared survey. In this paper, we describe the updated quasar candidate selection procedure, report the discovery of 16 quasars at 6.4 <= z <= 6.9 from an area of ∼13,020 deg², and present the quasar luminosity function (QLF) at z ∼ 6.7. The measured QLF follows Φ(L₁₄₅₀)∝L₁₄₅₀^–2.35 in the magnitude range -27.6 < M₁₄₅₀ < -25.5. We determine the quasar comoving spatial density at <z>= 6.7 and M₁₄₅₀ < -26.0 to be 0.39 ± 0.11 Gpc^–3 and find the exponential density evolution parameter to be k = -0.78 ± 0.18 from z ∼ 6 to z ∼ 6.7, corresponding to a rapid decline by a factor of ∼6 per unit redshift toward earlier epochs. This indicates that the rapid decline of quasar spatial density at z > 5 that was found by previous works continues to z > 6, at a rate significantly faster than the average decline rate between z ∼ 3 and 5. We measured quasar comoving emissivity at z ∼ 6.7, which indicates that high-redshift quasars are highly unlikely to make a significant contribution to hydrogen reionization. The broad absorption line quasar fraction at z >= 6.5 is measured to be >=22%. In addition, we also report the discovery of six additional quasars at z ∼ 6 in the Appendix.