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

TL;DR

Current surveys likely miss many lensed quasars at redshift above 6 due to observational biases, which impacts our understanding of early universe black hole growth and cosmic backgrounds.

Contribution

This paper predicts a large, overlooked population of lensed $z>6$ quasars and highlights how current selection methods fail to detect them, affecting cosmological inferences.

Findings

Many $z>6$ lensed quasars are undetected due to selection biases.

The undetected population could be as large as half or more of the total.

This hidden population influences the quasar luminosity function and cosmic background estimates.

Abstract

The discovery of the first strongly lensed $(\mu \approx 50)$ 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 $\mu \lesssim 10$ 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, $\beta$. For $\beta \lesssim 3.6$, we predict that the undetected lensed quasars could reach half of the population, whereas for $\beta \gtrsim 4.5$ 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.