A Giant Green Pea Identified in the Spectroscopy of Spatially Extended [OIII] Sources

Abstract

We present the results of the deep Subaru/FOCAS and Keck/MOSFIRE spectroscopy for four spatially extended [O III]$\lambda\lambda$4959,5007 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 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 $845\pm27$ \r{A} and an [O III] to [O II]$\lambda\lambda$3726,3729 ratio of [O III]/[O II]= $6.5\pm2.7$ that are as high as those of typical green peas (Cardamone et al. 2009). 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 that have extended [O III] emission with a type-2 AGN. OIIIB-4 is thus a giant green pea, which is a low stellar mass ($7\times10^7$ $M_\odot$) galaxy with a very high specific star formation rate (sSFR = $2\times10^2\, {\rm Gyr}^{-1}$), a high ionization parameter ($q_{ion} \sim 3\times10^8\, {\rm cm\,s^{-1}}$), and a low metallicity similar to those of green peas. Neither an AGN-light echo nor a fast radiative shock likely takes place due to the line diagnostics for spatially-resolved components of OIIIB-4 and no detections of He II$\lambda$4686 or [Ne V]$\lambda\lambda$3346,3426 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.