This is a resubmission of our approved proposal S17B-047, which was not achieved due to poor weather. We propose Keck/MOSIFRE spectroscopy for eight objects showing strong Spitzer/IRAC color excesses at z>5.5 to investigate statistical properties of ionizing sources for the first time at the epoch of reionization. Our targets include two plausible Pop-III or low-metal galaxy candidates with low [Oiii]5007/Hα ratio and large Lyα equivalent width of EW0>210 Å at zspec=5.7. These objects are newly identified in the Subaru/Hyper-Suprime Cam (HSC) survey and the Spitzer Large Area Survey with HSC, whose area is ×10 larger than previous observations. We will measure strength of high-ionization emission lines such as Civ1549, Heii1640, or Ciii]1909 for four spec-z confirmed targets at zspec=5.707−6.936, and confirm redshifts of four z>7 candidates by Lyα to prepare future observations investigating high-ionization emission lines in these galaxies. We will investigate a UV luminosity dependence of the ionizing photon production efficiency, ξion, and the presence of prominent Heii emission in the PopIII candidates, to discuss the contribution of each ionizing source (star-forming galaxies, AGNs, and Pop-III) to the reionization.
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