Most Lensed Quasars at z > 6 are Missed by Current Surveys

The discovery of the first strongly lensed (μ ≈ 50) quasar at z > 6 (J0439+1634) represents a breakthrough in our understanding of the early universe. We derive the theoretical consequences of the new discovery. We predict that the observed population of z > 6 quasars should contain many sources with magnifications μ ≲ 10 and with image separations below the resolution threshold. Additionally, current selection criteria could have missed a substantial population of lensed z > 6 quasars, due to the contamination of the drop-out photometric bands by lens galaxies. We argue that this predicted population of lensed z > 6 quasars would be misclassified and mixed up with low-z galaxies. We quantify the fraction of undetected quasars as a function of the slope of the bright end of the quasar luminosity function, β. For β ≲ 3.6, we predict that the undetected lensed quasars could reach half of the population, whereas for β ≳ 4.5 the vast majority of the z > 6 quasar population is lensed and still undetected. This would significantly affect the z > 6 quasar luminosity function and inferred black hole mass distributions, with profound implications for the ultraviolet, X-ray, and infrared cosmic backgrounds and the growth of early quasars.