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

Takashi Kojima, Masami Ouchi, Michael Rauch, Yoshiaki Ono, Kimihiko Nakajima, Yuki Isobe, Seiji Fujimoto, Yuichi Harikane, Takuya Hashimoto, Masao Hayashi, Yutaka Komiyama, Haruka Kusakabe, Ji Hoon Kim, Chien Hsiu Lee, Shiro Mukae, Tohru Nagao, Masato Onodera, Takatoshi Shibuya, Yuma Sugahara, Masayuki UmemuraKiyoto Yabe

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1 Citation (Scopus)

Abstract

We present element abundance ratios and ionizing radiation of local young low-mass (∼106 M o˙) extremely metal-poor galaxies (EMPGs) with a 2% solar oxygen abundance (O/H)o˙ 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)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)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 o˙. 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 o˙) stars, which may evolve into intermediate-mass black holes (⪆100 M o˙).

Original languageEnglish
Article number22
JournalAstrophysical Journal
Volume913
Issue number1
DOIs
Publication statusPublished - 2021 May 20

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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