We report the discovery of four quasars with M1450 ≳ −25.0 mag at z ∼ 5 and supermassive black hole mass measurement for one of the quasars. They were selected as promising high-redshift quasar candidates via deep learning and Bayesian information criterion, which are expected to be effective in discriminating quasars from the late-type stars and high-redshift galaxies. The candidates were observed by the Double Spectrograph on the Palomar 200-inch Hale Telescope. They show clear Lyα breaks at about 7000–8000 ˚A, indicating they are quasars at 4.7 < z < 5.6. For HSC J233107-001014, we measure the mass of its supermassive black hole (SMBH) using its Civ λ1549 emission line. The SMBH mass and Eddington ratio of the quasar are found to be ∼108 M⊙ and ∼0.6, respectively. This suggests that this quasar possibly harbors a fast growing SMBH near the Eddington limit despite its faintness (LBol < 1046 erg s−1). Our 100% quasar identification rate supports high efficiency of our deep learning and Bayesian information criterion selection method, which can be applied to future surveys to increase high-redshift quasar sample.

Abstract
1. INTRODUCTION
2. QUASAR SELECTION
    2.1. Photometric Data
    2.2. SED Models
    2.3. Quasar Selection Process
3. SPECTROSCOPY
4. RESULT
    4.1. Spectral Fitting
    4.2. BH Mass and Eddington Ratio
5. IMPLICATIONS ON QUASAR SELECTION
6. SUMMARY
ACKNOWLEDGMENTS
REFERENCES

• Suhyun Shin(SNU Astronomy Research Center, Astronomy Program, Dept. of Physics & Astronomy, Seoul National University)
• Myungshin Im(SNU Astronomy Research Center, Astronomy Program, Dept. of Physics & Astronomy, Seoul National University) Corresponding Author
• Yongjung Kim(Department of Astronomy and Atmospheric Sciences, College of Natural Sciences, Kyungpook National University, Kavli Institute for Astronomy and Astrophysics, Peking University)
• Linhua Jiang(Kavli Institute for Astronomy and Astrophysics, Peking University)