2019ApJ...871..199Y

We present the final results from our survey of luminous z ∼ 5.5 quasars. This is the first systematic quasar survey focusing on quasars at z ∼ 5.5, during the post-reionization epoch. It has been challenging to select quasars at 5.3 < z < 5.7 using conventional color selections, due to their similar optical colors to those of late-type stars, especially M dwarfs. We developed a new selection technique for z ∼ 5.5 quasars based on optical, near-infrared (IR), and mid-IR photometry, using data from the Sloan Digital Sky Survey (SDSS), PanSTARR1 (PS1), the UKIRT Infrared Deep Sky Surveys-Large Area Survey, the UKIRT Hemisphere Survey, the VISTA Hemisphere Survey, and the Wide Field Infrared Survey Explorer, covering ∼11,000 deg² of high galactic latitude sky. In this paper, we present the discovery of 15 new quasars at 5.22 <= z <= 5.78. Together with results from Yang et al., our survey provides a complete, flux-limited sample of 31 quasars at 5.3 <= z <= 5.7. We measure the quasar spatial density at z ∼ 5.5 and M₁₄₅₀ < -26.2. Our result is consistent with the rapid decline of the quasar spatial density from z = 5 to 6, with k = -0.66 ± 0.05 (ρ(z) ∝ 10^kz). In addition, we present a new survey using optical colors only from the full PS1 area for luminous quasars at z = 5.0-5.5, which is beyond the SDSS footprint, and report the preliminary results from this survey, including 51 new quasars discovered at 4.61 <= z <= 5.71.