A complete census of high redshift active galactic nuclei (AGNs) is crucial for understanding the growth history of super-massive black holes. However, it is widely known that previous wide-field AGN surveys are incomplete in terms of depth or sensitivities. For instance, optical broad-band surveys are insensitive to type-2 (narrow-line) AGNs since the luminosity of type-2 is not high enough to distinguish them from normal (non-AGN) star-forming galaxies. Here, we propose a trial to efficiently search for high redshift type-2 AGNs as well as type-1 in a wide field (∼ 1.6 deg2 in the COSMOS field) based on the “dual” narrow-band excess selection. By combining HSC NB718 and NB921 data, we have successfully selected 4 reliable NB718-NB921 dual-excess objects, that should be Lyα–CIV dual emitters (probably type-2 AGNs based on their equivalent width and/or their rest-frame UV luminosity) at z ∼ 4.9. The proposed follow-up optical spectroscopy with FOCAS will test how successfully this method can select high redshift faint AGNs, that enables us to establish a completely-new powerful method to search for high redshift faint AGNs in wide fields.
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