A Survey of z~6 Quasars in the SDSS Deep Stripe: I. a Flux-Limited Sample at z_{AB}<21

TL;DR

This study discovers and analyzes a flux-limited sample of z~6 quasars from SDSS, measuring their spatial density and luminosity function, and discusses their role in cosmic reionization.

Contribution

It presents the first flux-limited sample of z~6 quasars at fainter magnitudes and determines their luminosity function and spatial density, providing new insights into early universe quasar populations.

Findings

Quasars at z~6 are 1-2 magnitudes fainter than previous samples.

The spatial density of z~6 quasars is approximately 5.0 x 10^{-9} Mpc^{-3} mag^{-1}.

The bright-end quasar luminosity function has a steep slope of -3.1, indicating fewer luminous quasars at this epoch.

Abstract

We present the discovery of five quasars at z~6 selected from 260 deg^2 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<M_{1450}<-25.4 (H_0=70 km s^{-1} Mpc^{-1}, Omega_{m}=0.3, and Omega_{Lambda}=0.7). We use the 1/V_{a} method to determine that the comoving quasar spatial density at <z>=6.0 and <M_{1450}>=-25.8 is (5.0+/-2.1) x 10^{-9} Mpc^{-3} mag^{-1}. We model the bright-end quasar luminosity function (QLF) at z~6 as a power law Phi(L_{1450}) \propto L_{1450}^{beta}. The slope beta 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.