The Infrared Medium-deep Survey. VI. Discovery of Faint Quasars at z ∼ 5 with a Medium-band-based Approach

The faint quasars with M ₁₄₅₀ > -24 mag are known to hold the key to the determination of the ultraviolet emissivity for the cosmic reionization. But only a few have been identified so far because of the limitations on the survey data. Here we present the first results of the z ∼ 5 faint quasar survey with the Infrared Medium-deep Survey (IMS), which covers ∼100 deg² areas in J band to the depths of J _AB ∼ 23 mag. To improve selection methods, the medium-band follow-up imaging has been carried out using the SED camera for QUasars in Early uNiverse (SQUEAN) on the Otto Struve 2.1 m Telescope. The optical spectra of the candidates were obtained with 8 m class telescopes. We newly discovered 10 quasars with -25 < M ₁₄₅₀ < -23 at z ∼ 5, among which three have been missed in a previous survey using the same optical data over the same area, implying the necessity for improvements in high-redshift faint quasar selection. We derived photometric redshifts from the medium-band data and found that they have high accuracies of < | {{Δ }}z| /(1+z)> =0.016. The medium-band-based approach allows us to rule out many of the interlopers that contaminate ≳20% of the broadband-selected quasar candidates. These results suggest that the medium-band-based approach is a powerful way to identify z ∼ 5 quasars and measure their redshifts at high accuracy (1%-2%). It is also a cost-effective way to understand the contribution of quasars to the cosmic reionization history.