A Survey of z > 5.8 quasars in the Sloan Digital Sky Survey I: Discovery of three new quasars and the spatial density of luminous quasars at z ~ 6

TLDR: In this article, the authors present the results from a survey of i-dropout objects selected from ~1550 deg2 of multicolor imaging data from the Sloan Digital Sky Survey to search for luminous quasars at z 5.8.

Abstract: We present the results from a survey of i-dropout objects selected from ~1550 deg2 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* 0.90. The ARC 3.5 m spectrum of SDSSp J103027.10+052455.0 shows that over a range of ~300 A 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 A 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 M1450 < -26.8 (H0 = 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 109 M⊙ by the Eddington argument, with likely dark halo masses on the order of 1013 M⊙. Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift.