Empress. II. highly Fe-enriched metal-poor galaxies with ∼ 1.0 (Fe/O)⊙ and 0.02 (O/H)⊙: Possible traces of super massive (> 300M⊙) stars in early galaxies

We present element abundance ratios and ionizing radiation of local young low-mass (∼ 10⁶M⊙) extremely metal poor galaxies (EMPGs) with a 2% solar oxygen abundance (O/H)⊙ and a high specific star-formation rate (sSFR∼300 Gyr⁻¹), and other (extremely) metal poor galaxies, which are compiled from Extremely Metal-Poor Representatives Explored by the Subaru Survey (EMPRESS) and the literature. Weak emission lines such as [Fe iii]4658 and He ii4686 are detected in very deep optical spectra of the EMPGs taken with 8m-class telescopes including Keck and Subaru (Kojima et al. 2019; Izotov et al. 2018), enabling us to derive element abundance ratios with photoionization models. We find that neon- and argon-to-oxygen ratios are comparable to those of known local dwarf galaxies, and that the nitrogen-to-oxygen abundance ratios (N/O) are lower than 20% (N/O)⊙ consistent with the low oxygen abundance. However, the iron-to-oxygen abundance ratios (Fe/O) of the EMPGs are generally high; the EMPGs with the 2%-solar oxygen abundance show high Fe/O ratios of ∼ 90−140% (Fe/O)⊙, which are unlikely explained by suggested scenarios of Type Ia supernova iron productions, iron’s dust depletion, and metal-poor gas inflow onto previously metal-riched galaxies with solar abundances. Moreover, these EMPGs have very high He ii4686/Hβ ratios of ∼1/40, which are not reproduced by existing models of high-mass X-ray binaries whose progenitor stellar masses are less than 120M⊙. Comparing stellar-nucleosynthesis and photoionization models with a comprehensive sample of EMPGs identified by this and previous EMPG studies, we propose that both the high Fe/O ratios and the high He ii4686/Hβ ratios are explained by the past existence of super massive (> 300M⊙) stars, which may evolve into intermediate-mass black holes (& 100M⊙).