The quasar luminosity function at $z\sim5$ via deep learning and Bayesian information criterion

TL;DR

This study uses deep learning and Bayesian information criterion to identify faint quasars at high redshift, extending the luminosity function to fainter magnitudes and providing insights into their role in early universe ionization.

Contribution

Introduces a novel quasar selection method combining deep learning and Bayesian information criterion, improving efficiency and reducing contamination in high-redshift quasar surveys.

Findings

Extended quasar luminosity function to $M_{1450} \\sim -22$ mag at $z\\sim5$

Achieved 83% efficiency in quasar selection with low galaxy contamination

Faint-end slope of the quasar luminosity function is approximately -1.6

Abstract

Understanding the faint end of quasar luminosity function at a high redshift is important since the number density of faint quasars is a critical element in constraining ultraviolet (UV) photon budgets for ionizing the intergalactic medium (IGM) in the early universe. Here, we present quasar LF reaching $M_{1450} \sim -22.0$ AB mag at $z\sim5$, about one magnitude deeper than previous UV LFs. We select quasars at $z\sim5$ with a deep learning technique from deep data taken by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), covering a 15.5 deg$^2$ area. Beyond the traditional color selection method, we improved the quasar selection by training an artificial neural network for distinguishing $z\sim5$ quasars from non-quasar sources based on their colors and adopting the Bayesian information criterion that can further remove high-redshift galaxies from the quasar sample. When applied to a small sample of spectroscopically identified quasars and galaxies, our method is successful in selecting quasars at $\sim83 \%$ efficiency ($5/6$) while minimizing the contamination rate of high-redshift galaxies ($1/8$) by up to three times compared to the selection using color selection alone ($3/8$). The number of our final quasar candidates with $M_{1450} < -22.0$ mag is 35. Our quasar UV LF down to $M_{1450} = -22$ mag or even fainter ($M_{1450} = -21$ mag) suggests a rather low number density of faint quasars and the faint-end slope of $-1.6^{+0.21}_{-0.19}$, favoring a scenario where quasars play a minor role in ionizing the IGM at high redshift.