Observations of the first-generation galaxies are keys to understand the metal enrichment of the universe and the process of cosmic reionization. Vigorous efforts have been made to find evidence of Population-III (Pop-III) stars in high-redshift galaxies at z∼6−7, yet no clear signatures have been found. We propose to take a deep Keck/MOSFIRE spectrum of HSC J1423-0018, a Pop-III galaxy candidate at z=6.13, which was identified through our search of z>6 low-luminosity quasars with the Subaru/Hyper Suprime-Cam covering ∼650 deg2. Its Lyα line width of FWHM<230 km s−1 is too narrow to be a type-I quasar and rather similar to that of a Lyα emitter (LAE). The rest-frame equivalent width of Lyα based on its discovery spectrum is exceptionally high (EW0(Lyα)=370±30Å), and its Lyα luminosity (logLLyα=44.30±0.01 erg s−1) is higher than known z>6 LAEs. In this program, we aim at identifying Heii λ1640 emission line of the target as a signature of a Pop-III galaxy for the first time, as well as other high ionization lines (Civ λ 1549, Oiii] λλ 1660 1666), taking advantage of the target brightness. A line diagnosis using Lyα, Civ, and Heii is used to examine the classification, namely a Pop-III/II galaxy or a type-II quasar.
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