The Pan-STARRS1 z > 5.6 Quasar Survey. III. The z & AP; 6 Quasar Luminosity Function

We present the z asymptotic to 6 type-1 quasar luminosity function (QLF), based on the Pan-STARRS1 (PS1) quasar survey. The PS1 sample includes 125 quasars at z asymptotic to 5.7-6.2, with -28 ? M-1450 ? -25. With the addition of 48 fainter quasars from the SHELLQs survey, we evaluate the z asymptotic to 6 QLF over -28 ? M-1450 ? -22. Adopting a double power law with an exponential evolution of the quasar density (Phi(z) proportional to 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 alpha = - 1.70(-0.19)(+0.29), and a steep bright-end slope of beta = - 3.84(-1.21)(+0.63). Based on our new QLF model, we determine the quasar comoving spatial density at z asymptotic to 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 asymptotic to -0.7, from z asymptotic to 7 to z asymptotic to 4. Additionally, we derive an ionizing emissivity of is an element of(912)(z = 6) = 7.23(-1.02)(+1.65) x 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 asymptotic to 6, we calculate an H I photoionizing rate of gamma(H I)(z = 6) asymptotic to 6 x 10-16 s(-1), strongly disfavoring a dominant role of quasars in hydrogen reionization.