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We present the discovery of five quasars at z~6 selected from 260 deg² of the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the Celestial Equator. The five quasars with 20=6. 0 and =-25. 8 is (5. 0+/-2. 1) x 10^-9 Mpc^-3 mag^-1. We model the bright-end quasar luminosity function (QLF) at z~6 as a power law Phi (L₁₄₅₀) L₁₄₅₀^beta. The slope beta calculated from a combination of our sample and the luminous SDSS quasar sample is -3. 1+/-0. 4, significantly steeper than the slope of the QLF at z~4. Based on the derived QLF, we find that the quasar/AGN population cannot provide enough photons to ionize the intergalactic medium (IGM) at z~6 unless the IGM is very homogeneous and the luminosity (L*₁₄₅₀) at which the QLF power law breaks is very low.
Jiang et al. (Thu,) studied this question.