We present the results of the deep Subaru/FOCAS and Keck/MOSFIRE spectroscopy for four spatially extended [O iii] λλ4959, 5007 sources, dubbed [O iii] blobs, at z = 0.6-0.8 that are originally pinpointed by large-area Subaru imaging surveys. The line diagnostics of the rest-frame optical lines suggests that only one [O iii] blob, OIIIB-3, presents an active galactic nucleus (AGN) signature, indicating that hot gas of the rest of the [O iii] blobs is heated by star formation. One of such star-forming [O iii] blobs, OIIIB-4, at z = 0.838 has an [O iii] equivalent width of 845 ±27 Å and an [O iii]-to-[O ii] λλ3726, 3729 ratio of [O iii]/[O ii] = 6.5 ±2.7, which are as high as those of typical Green Peas. The spatially resolved spectrum of OIIIB-4 shows [O iii]/[O ii] = 5-10 over 14 kpc in the entire large [O iii] extended regions of OIIIB-4, unlike the known Green Peas, whose strong [O iii] emission region is compact. Moreover, OIIIB-4 presents no high-ionization emission lines, unlike Green Beans, which have extended [O iii] emission with a type 2 AGN. OIIIB-4 is thus a giant Green Pea, which is a low stellar mass (7 ×107 ) galaxy with a very high specific star formation rate (sSFR = 2 ×102 Gyr-1), a high-ionization parameter (q ion ∼ 3 ×108 cm s-1), and a low metallicity similar to those of Green Peas. Neither an AGN light echo nor a fast radiative shock likely takes place owing to the line diagnostics for spatially resolved components of OIIIB-4 and no detections of He ii λ4686 or [Ne v] λλ3346, 3426 lines that are fast radiative shock signatures. There is a possibility that the spatially extended [O iii] emission of OIIIB-4 is originated from outflowing gas produced by the intense star formation in a density-bounded ionization state.
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