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

초록

We present the result of the Infrared Medium-deep Survey (IMS) z similar to 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 deg(2). 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 similar to 6. The follow-up spectroscopic observations for the remaining four candidates were carried out, and we confirmed that two out of four are z similar to 6 quasars. With this complete sample, we revisited the quasar space density at z similar to 6 down to M (1450) similar to -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.

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