Newly discovered $z\sim 5$ quasars based on deep learning and Bayesian information criterion

We report the discovery of four quasars with $M_{1450}\gtrsim -25.0$ mag at $z\sim 5$ 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 Ly$\alpha$ breaks at about 7000-8000 Å, 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 C\Romannum{4}$\lambda 1549$ emission line. The SMBH mass and Eddington ratio of the quasar are found to be $\sim {10}^8{M}_{\odot }$ and $\sim 0.6$, respectively. This suggests that this quasar possibly harbors a fast growing SMBH near the Eddington limit despite its faintness ($L_{\mathrm{B}\mathrm{o}\mathrm{l}}<{10}^{46}$ erg s${}^{-1}$). Our 100 $\mathrm{\%}$ 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.