A SURVEY OF z ∼ 6 QUASARS IN THE SLOAN DIGITAL SKY SURVEY DEEP STRIPE. II. DISCOVERY OF SIX QUASARS AT zAB>21

We present the discovery of six new quasars at z ∼ 6 selected from the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The six quasars are about 2 mag fainter than the luminous z ∼ 6 quasars found in the SDSS main survey and 1 mag fainter than the quasars reported in Paper I. Four of them comprise a complete flux-limited sample at 21 < zAB < 21.8 over an effective area of 195 deg2. The other two quasars are fainter than zAB = 22 and are not part of the complete sample. The quasar luminosity function at z ∼ 6 is well described as a single power law Φ(L1450) ∝ Lβ1450 over the luminosity range −28 < M1450 < −25. The best-fitting slope β varies from −2.6 to −3.1, depending on the quasar samples used, with a statistical error of 0.3–0.4. About 40% of the quasars discovered in the SDSS southern survey have very narrow Lyα emission lines, which may indicate small black hole masses and high Eddington luminosity ratios, and therefore short black hole growth timescales for these faint quasars at early epochs.

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