2008AJ....135.1057J

We present the discovery of five quasars at z ∼ 6 selected from 260deg² 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<z_AB<21 are 1-2 magnitudes fainter than the luminous z ∼ 6 quasars discovered in the SDSS main survey. One of them was independently discovered by the UKIRT Infrared Deep Sky Survey. These quasars, combined with another z ∼ 6 quasar known in this region, make a complete flux-limited quasar sample at z_AB<21. The sample spans the redshift range 5.85 ≤ z ≤ 6.12 and the luminosity range -26.5≤M₁₄₅₀≤-25.4 (H₀=70km.s^–1.Mpc^–1, Ω_m=0.3, and Ω_Λ=0.7). We use the 1/V_a method to determine that the comoving quasar spatial density at <z≥6.0 and <M₁₄₅₀≥-25.8 is (5.0±2.1)x10^–9.Mpc^–3.mag^–1. We model the bright-end quasar luminosity function (QLF) at z ∼ 6 as a power law Φ(L₁₄₅₀) ∝ L^β₁₄₅₀. The slope β 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/active galactic nucleus (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.