A Survey of [CLC][ITAL]z[/ITAL][/CLC] > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at [CLC][ITAL]z[/ITAL][/CLC] ∼ 6

Abstract

We present the results from a survey of i -dropout objects selected from ∼1550 deg 2 of multicolor imaging data from the Sloan Digital Sky Survey to search for luminous quasars at z ≳ 5.8. Objects with i *- z * > 2.2 and z * < 20.2 are selected, and follow-up J -band photometry is used to separate L- and T-type cool dwarfs from high-redshift quasars. We describe the discovery of three new quasars, SDSSp J083643.85+005453.3 ( z = 5.82), J130608.26+035626.3 ( z = 5.99), and J103027.10+052455.0 ( z = 6.28). The quasar SDSSp J083643.85+005453.3 is a radio source with flux of 1.1 mJy at 20 cm. The spectra of all three quasars show strong and broad Lyα + N V emission lines and very strong Lyα forest absorption, with a mean continuum decrement D A > 0.90. The ARC 3.5 m spectrum of SDSSp J103027.10+052455.0 shows that over a range of ∼300 Å immediately blueward of the Lyα emission, the average transmitted flux is only 0.003 ± 0.020 times that of the continuum level, consistent with zero flux over a ∼300 Å range of the Lyα forest region and suggesting a tentative detection of the complete Gunn-Peterson trough. The existence of strong metal lines in the quasar spectra suggests early metal enrichment in the quasar environment. The three new objects, together with the previously published z = 5.8 quasar SDSSp J104433.04-012502.2, form a complete color-selected flux-limited sample at z ≳ 5.8. We estimate the selection function of this sample, taking into account the estimated variations in the quasar spectral energy distribution, as well as observational photometric errors. We find that at z = 6, the comoving density of luminous quasars at M 1450 < -26.8 ( H 0 = 50 km s -1 Mpc -1 , Ω = 1) is 1.1 × 10 -9 Mpc -3 . This is a factor of ∼2 lower than that at z ∼ 5 and is consistent with an extrapolation of the observed quasar evolution at z < 5. Using the current sample, we discuss the constraint on the shape of the quasar luminosity function and the implications for the contribution of quasars to the ionizing background at z ∼ 6. The luminous quasars discussed in the paper have central black hole masses of several times 10 9 M ⊙ by the Eddington argument, with likely dark halo masses on the order of 10 13 M ⊙ . Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift.