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We present element abundance ratios and ionizing radiation of local younglow-mass (~$10^{6}$ M_sun) extremely metal poor galaxies (EMPGs) with a 2%solar oxygen abundance (O/H)_sun and a high specific star-formation rate(sSFR~300 Gyr$^{-1}$), and other (extremely) metal poor galaxies, which arecompiled from Extremely Metal-Poor Representatives Explored by the SubaruSurvey (EMPRESS) and the literature. Weak emission lines such as [FeIII]4658and HeII4686 are detected in very deep optical spectra of the EMPGs taken with8m-class telescopes including Keck and Subaru (Kojima et al. 2019, Izotov etal. 2018), enabling us to derive element abundance ratios with photoionizationmodels. We find that neon- and argon-to-oxygen ratios are comparable to thoseof known local dwarf galaxies, and that the nitrogen-to-oxygen abundance ratios(N/O) are lower than 20% (N/O)_sun consistent with the low oxygen abundance.However, the iron-to-oxygen abundance ratios (Fe/O) of the EMPGs are generallyhigh; the EMPGs with the 2%-solar oxygen abundance show high Fe/O ratios of~90-140% (Fe/O)_sun, which are unlikely explained by suggested scenarios ofType Ia supernova iron productions, iron's dust depletion, and metal-poor gasinflow onto previously metal-riched galaxies with solar abundances. Moreover,these EMPGs have very high HeII4686/H$\beta$ ratios of ~1/40, which are notreproduced by existing models of high-mass X-ray binaries whose progenitorstellar masses are less than 120 M_sun. Comparing stellar-nucleosynthesis andphotoionization models with a comprehensive sample of EMPGs identified by thisand previous EMPG studies, we propose that both the high Fe/O ratios and thehigh HeII4686/H$\beta$ ratios are explained by the past existence of supermassive ($>$300 M_sun) stars, which may evolve into intermediate-mass blackholes ($\gtrsim$100 M_sun).

astrophysical journal/the astrophysical journal

EMPRESS. II. Highly Fe-Enriched Metal-poor Galaxies with $\sim 1.0$  (Fe/O)$_\odot$ and $0.02$ (O/H)$_\odot$ : Possible Traces of Super Massive  ($>300 M_{\odot}$) Stars in Early Galaxies