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 $6.4lesssim z lesssim6.9$ from area of $sim$13,020 deg$^2$, and present the quasar luminosity function (QLF) at $zsim6.7$. 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 $rm 0.39pm0.11 Gpc^{-3}$ and find that the exponential density evolution parameter to be $k=-0.78pm0.18$ from $zsim6$ to $zsim6.7$, corresponding to a rapid decline by a factor of $sim 6$ per unit redshift towards earlier epoch, a rate significantly faster than that at $zsim 3- 5$. The cosmic time between $zsim6$ and $zsim6.7$ 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 $zsim6.7$ 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 $zgtrsim6.5$ is measured to be $gtrsim$22%. In addition, we also report the discovery of additional five quasars at $zsim6$ in the appendix.
تحميل البحث