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 119 quasars at z ≈ 5.7 − 6.2 with −28 􏰃 M1450 􏰃 −25. Complemented by 48 fainter quasars from the SHELLQs survey, we evaluate the z ≈ 6 QLF over −28 􏰃 M1450 􏰃 −22. Adopting a double power law with an exponential evolution of the quasar density (Φ(z) ∝ 10k(z−6); k = −0.7), we use a maximum likelihood method to model our data. We find a break magnitude of M∗ = −26.59+0.92 mag, a faint end slope of α = −1.77+0.30, and a steep bright end −0.53 −0.17 slope with β = −4.04+0.80. Based on our new QLF model we determine the quasar comoving spatial −1.49 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 < −26) = 1.15+0.14 cGpc−3. In comparison with the literature, we find 1450 −0.13 density at z≈6 to be n(M the quasar density to evolve with a constant value of k ≈ −0.7 from z ≈ 7 to z ≈ 4. Additionally, (z = 6) = 7.44+1.84 × 1022 erg s−1Hz−1cMpc−3 based on the 912 −1.17 we derive an ionizing emissivity of ε QLF measurement. Given standard assumptions and the recent measurement of the mean free path of Becker et al. (2021) at z ≈ 6 we calculate an HI photoionizing rate of ΓHI(z=6) ≈ 6 × 10−16 s−1, strongly disfavoring a dominant role of quasars in hydrogen reionization.