EMPRESS. II. Highly Fe-enriched Metal-poor Galaxies with ∼1.0 (Fe/O)_⊙ and 0.02 (O/H)_⊙: Possible Traces of Supermassive (>300 M_⊙) Stars in Early Galaxies

We present element abundance ratios and ionizing radiation of local young low-mass (∼10⁶ M _⊙) 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] λ4658 and He ii λ4686 are detected in very deep optical spectra of the EMPGs taken with 8 m class telescopes, including Keck and Subaru, enabling us to derive element abundance ratios with photoionization models. We find that neon-to-oxygen 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 to be 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, the EMPGs with the 2%-solar oxygen abundance have very high He ii λ4686/Hβ ratios of ∼1/40, which are not reproduced by existing models of high-mass X-ray binaries with progenitor stellar masses <120 M _⊙. 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 ii λ4686/Hβ ratios are explained by the past existence of supermassive (>300 M _⊙) stars, which may evolve into intermediate-mass black holes (≳100 M _⊙).