exploring reionization-era quasars III: Discovery of 16 quasars at 6.4 . z . 6.9 with desi legacy imaging surveys and ukirt hemisphere survey and quasar luminosity function at z ∼ 6.7

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 . z . 6.9 from 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 hzi=6.7 and M₁₄₅₀ < −26.0 to be 0.39 ± 0.11Gpc⁻³ and find that 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 towards earlier epoch, a rate significantly faster than that at z ∼ 3 − 5. The cosmic time between z ∼ 6 and z ∼ 6.7 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 ∼ 6.7 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 & 6.5 is measured to be &22%. In addition, we also report the discovery of additional five quasars at z ∼ 6 in the appendix.