We propose detailed spectroscopic follow-up for two extreme [Oiii] emitters at zspec=0.828 and 0.728 whose [Oiii]5007 equivalent widths (EWs) are ∼5000 Å, highest among objects reported to date. These emitters are identified in our unique ∼650 deg2 Subaru/Hyper-Suprime Cam Survey. The EWs of these two emitters can not be reproduced by classical stellar population synthesis models, and hard ionizing spectra (such as produced by massive star binary population) are needed. By taking deep optical and near-infrared spectra using Subaru/FOCAS and MOIRCS, respectively, we will archive the following three science goals: 1) Investigating the existence of hidden AGN activities through the BPT diagram. 2) Determining metallicities using the [Oiii]4363 line. 3) Measuring the hardness of the ionizing spectra with the Hβ, Hei5876, and Heii4686 line ratios. Based on these results, we will discuss whether the new binary population model can explain the properties of these two extreme [Oiii] emitters, or modification of our knowledge of the stellar population model is needed. This program is important verification of the current stellar population models, independent to previous studies based on Heii emitters, and crucial for understanding cosmic reionization sources.
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