2019MNRAS.488.1035K

Determinations of the ultraviolet (UV) luminosity function of active galactic nuclei (AGN) at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of data. We address this problem by constructing a sample of more than 80 000 colour-selected AGN from redshift z = 0 to 7.5 using multiple data sets homogenized to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift z = 0 to 7.5. The luminosity function has a double power-law form at all redshifts. The break magnitude M_* shows a steep brightening from M_* ∼ -24 at z = 0.7 to M_* ∼ -29 at z = 6. The faint-end slope β significantly steepens from -1.9 at z < 2.2 to -2.4 at z ≃ 6. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at z ∼ 6 is subdominant (<3 per cent), although it can be non-negligible (∼10 per cent) if these luminosity functions hold down to M₁₄₅₀ = -18. Under reasonable assumptions, AGN can reionize He II by redshift z = 2.9. At low redshifts (z < 0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of H I absorption lines in the intergalactic medium. Our analysis also reveals important systematic errors in the data, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, codes, and data publicly available.