. 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 deg, and present the quasar luminosity function (QLF) at z ∼ 6.7. The measured QLF follows Φ(L1450) ∝ L−2.35 1450 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 ± 0.11Gpc−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 Corresponding author: Feige Wang fgwang@physics.ucsb.edu ar X iv :1 81 0. 11 92 6v 1 [ as tr oph .G A ] 2 9 O ct 2 01 8