The Pan-STARRS1 z > 5.6 Quasar Survey. III. The z ≈ 6 Quasar Luminosity Function

We present the z ≈ 6 type-1 quasar luminosity function (QLF), based on the Pan-STARRS1 (PS1) quasar survey. The PS1 sample includes 125 quasars at z ≈ 5.7-6.2, with −28 ≲ M ₁₄₅₀ ≲ −25. With the addition of 48 fainter quasars from the SHELLQs survey, we evaluate the z ≈ 6 QLF over −28 ≲ M ₁₄₅₀ ≲ −22. Adopting a double power law with an exponential evolution of the quasar density (Φ(z) ∝ 10^k(z−6); k = −0.7), we use a maximum likelihood method to model our data. We find a break magnitude of M * = − 26.38 − 0.60 + 0.79 mag , a faint-end slope of α = − 1.70 − 0.19 + 0.29 , and a steep bright-end slope of β = − 3.84 − 1.21 + 0.63 . Based on our new QLF model, we determine the quasar comoving spatial density at z ≈ 6 to be n ( M 1450 < − 26 ) = 1.16 − 0.12 + 0.13 cGpc − 3 . In comparison with the literature, we find the quasar density to evolve with a constant value of k ≈ −0.7, from z ≈ 7 to z ≈ 4. Additionally, we derive an ionizing emissivity of ϵ 912 ( z = 6 ) = 7.23 − 1.02 + 1.65 × 10 22 erg s − 1 Hz − 1 cMpc − 3 , based on the QLF measurement. Given standard assumptions, and the recent measurement of the mean free path by Becker et al. at z ≈ 6, we calculate an H i photoionizing rate of Γ_{H I}(z = 6) ≈ 6 × 10⁻¹⁶ s⁻¹, strongly disfavoring a dominant role of quasars in hydrogen reionization.