The quasar luminosity function at $z\sim 5$ via deep learning and Bayesian information criterion

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 $z\sim 5$, about one magnitude deeper than previous UV LFs. We select quasars at $z\sim 5$ 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 $z\sim 5$ 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 $\sim 83\%$ 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 $-1.6_{-0.19}^{+0.21}$, favoring a scenario where quasars play a minor role in ionizing the IGM at high redshift.

19 pages, 8 figures, Accepted for publication in ApJ

References in corpus (17)

• The Cosmic Evolution Survey (COSMOS) -- Overview
• astroquery: An Astronomical Web-Querying Package in Python
• Weak Gravitational Lensing with COSMOS: Galaxy Selection and Shape Measurements
• The Pan-STARRS1 distant z>5.6 quasar survey: more than 100 quasars within the first Gyr of the universe
• Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe
• Physical properties of 15 quasars at $z\gtrsim 6.5$
• The first ultraviolet quasar stacked spectrum at z=2.4 from WFC3
• A survey of luminous high-redshift quasars with SDSS and WISE II. the bright end of the quasar luminosity function at z ~ 5
• Efficient Selection of Quasar Candidates Based on Optical and Infrared Photometric Data Using Machine Learning
• On the AGN nature of two UV bright sources at z_spec~5.5 in the CANDELS fields: an update of the AGN space density at M1450~-22.5
• The Extremely Luminous Quasar Survey in the Pan-STARRS 1 Footprint (PS-ELQS)
• Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function
• The rapid transition from star-formation to AGN dominated rest-frame UV light at z ~ 4
• Massive Lyman Break Galaxies at z~3 in the Spitzer Extragalactic First Look Survey
• ELDAR, a new method to identify AGN in multi-filter surveys: the ALHAMBRA test-case
• Pure Density Evolution of the Ultraviolet Quasar Luminosity Function at $2\lesssim z\lesssim 6$
• The Infrared Medium-deep Survey. III. Survey of Luminous Quasars at 4.7 $\le$ z $\le$ 5.4