A survey of luminous high-redshift quasars with SDSS and WISE II. the bright end of the quasar luminosity function at z ~ 5

TL;DR

This study presents a comprehensive survey of luminous quasars at redshift around 5, refining the quasar luminosity function's bright end, and assessing their contribution to cosmic reionization.

Contribution

It provides the largest uniform sample of z~5 quasars, models the bright end of the luminosity function with improved parameters, and explores quasar evolution and ionizing contribution.

Findings

Bright end slope of the QLF is flatter than previous estimates.

Quasars contribute 18-45% to the ionizing background at z~5.

Evidence of evolution in radio loud fraction with luminosity.

Abstract

This is the second paper in a series on a new luminous z ~ 5 quasar survey using optical and near-infrared colors. Here we present a new determination of the bright end of the quasar luminosity function (QLF) at z ~ 5. Combined our 45 new quasars with previously known quasars that satisfy our selections, we construct the largest uniform luminous z ~ 5 quasar sample to date, with 99 quasars in the range 4.7 <= z < 5.4 and -29 < M1450 <= -26.8, within the Sloan Digital Sky Survey (SDSS) footprint. We use a modified 1/Va method including flux limit correction to derive a binned QLF, and we model the parametric QLF using maximum likelihood estimation. With the faint-end slope of the QLF fixed as alpha = -2.03 from previous deeper samples, the best fit of our QLF gives a flatter bright end slope beta = -3.58+/-0.24 and a fainter break magnitude M*1450 = -26.98+/-0.23 than previous studies at similar redshift. Combined with previous work at lower and higher redshifts, our result is consistent with a luminosity evolution and density evolution (LEDE) model. Using the best fit QLF, the contribution of quasars to the ionizing background at z ~ 5 is found to be 18% - 45% with a clumping factor C of 2 - 5. Our sample suggests an evolution of radio loud fraction with optical luminosity but no obvious evolution with redshift.