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

Abstract

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 , a faint-end slope of , and a steep bright-end slope of . Based on our new QLF model, we determine the quasar comoving spatial density at z ≈ 6 to be . 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 , 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 ΓH I(z = 6) ≈ 6 × 10−16 s−1, strongly disfavoring a dominant role of quasars in hydrogen reionization.