EMPRESS. IV. Extremely Metal-poor Galaxies Including Very Low-mass Primordial Systems with M = 104–105 M ⊙ and 2%–3% (O/H): High (Fe/O) Suggestive of Metal Enrichment by Hypernovae/Pair-instability Supernovae

• Published in: The Astrophysical journal 2022-02, Vol.925 (2)
• Main Authors: Isobe, Yuki, Ouchi, Masami, Suzuki, Akihiro, Moriya, Takashi J., Nakajima, Kimihiko, Nomoto, Ken’ichi, Rauch, Michael, Harikane, Yuichi, Kojima, Takashi, Ono, Yoshiaki, Fujimoto, Seiji, Inoue, Akio K., Kim, Ji Hoon, Komiyama, Yutaka, Kusakabe, Haruka, Lee, Chien-Hsiu, Maseda, Michael, Matthee, Jorryt, Michel-Dansac, Leo, Nagao, Tohru, Nanayakkara, Themiya, Nishigaki, Moka, Onodera, Masato, Sugahara, Yuma, Xu, Yi
• Format: Article
• Language: English
• Subjects: Astrophysics; Dwarf galaxies; Galaxy chemical evolution; Galaxy formation; Galaxy structure; Sciences of the Universe; Star formation
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac3509

We present Keck/LRIS follow-up spectroscopy for 13 photometric candidates of extremely metal-poor galaxies (EMPGs) selected by a machine-learning technique applied to the deep (~26 AB mag) optical and wide-area (~500 deg2) Subaru imaging data in the EMPRESS survey. Nine out of the 13 candidates are EMPGs with an oxygen abundance (O/H) less than ~10% solar value (O/H)⊙, and four sources are contaminants of moderately metal-rich galaxies or no emission-line objects. Notably, two out of the nine EMPGs have extremely low stellar masses and oxygen abundances of 5 × 104-7 × 105 M ⊙ and 2%-3% (O/H)⊙, respectively. With a sample of five EMPGs with (Fe/O) measurements, two (three) of which are taken from this study (the literature), we confirm that two EMPGs with the lowest (O/H) ratios of ~2% (O/H)⊙ show high (Fe/O) ratios of ~0.1, close to the solar abundance ratio. Comparing galaxy chemical enrichment models, we find that the two EMPGs cannot be explained by a scenario of metal-poor gas accretion/episodic star formation history due to their low (N/O) ratios. We conclude that the two EMPGs can be reproduced by the inclusion of bright hypernovae and/or hypothetical pair-instability supernovae (SNe) preferentially produced in a metal-poor environment. This conclusion implies that primordial galaxies at z ~ 10 could have a high abundance of Fe that did not originate from Type Ia SNe with delays and that Fe may not serve as a cosmic clock for primordial galaxies.