The Infrared Medium-deep Survey. IX. Discovery of Two New z ∼ 6 Quasars and Space Density Down to M ₁₄₅₀ ∼ −23.5 mag

We present the result of the Infrared Medium-deep Survey (IMS) z ∼ 6 quasar survey, using the combination of the IMS near-infrared images and the Canada-France-Hawaii Telescope Legacy Survey optical images. The traditional color selection method results in 25 quasar candidates over 86 deg². We introduce the corrected Akaike information criterion (AICc) with the high-redshift quasar and late-type star models to prioritize the candidates efficiently. Among the color-selected candidates, seven plausible candidates finally passed the AICc selection, of which three are known quasars at z ∼ 6. The follow-up spectroscopic observations for the remaining four candidates were carried out, and we confirmed that two out of four are z ∼ 6 quasars. With this complete sample, we revisited the quasar space density at z ∼ 6 down to M ₁₄₅₀ ∼ −23.5 mag. Our result supports the low quasar space density at the luminosity where the quasar’s ultraviolet ionizing emissivity peaks, favoring a minor contribution of quasars to the cosmic reionization.