A Survey of Z ∼ 6 Quasars in the Sdss Deep Stripe : I . a Flux - Limited Sample at Z Ab < 21

We present the discovery of five quasars at z ∼ 6 selected from 260 deg 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 < zAB < 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 zAB < 21. The sample spans the redshift range 5.85 ≤ z ≤ 6.12 and the luminosity range −26.5 ≤ M1450 ≤ −25.4 (H0 = 70 km s Mpc, Ωm = 0.3, and ΩΛ = 0.7). We use the 1/Va method to determine that the comoving quasar spatial density at 〈z〉 = 6.0 and 〈M1450〉 = −25.8 is (5.0 ± 2.1) × 10 Mpc mag. We model the bright-end quasar luminosity function (QLF) at z ∼ 6 as a power law Φ(L1450) ∝ L β 1450. 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/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. Subject headings: galaxies: active — quasars: emission lines — quasars: general