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

Yuki Isobe , Masami Ouchi , Akihiro Suzuki , Takashi J. Moriya , Kimihiko Nakajima , Ken'Ichi Nomoto , Michael Rauch , Yuichi Harikane , Takashi Kojima , Yoshiaki Ono , Seiji Fujimoto , Akio K. Inoue , Ji Hoon Kim , Yutaka Komiyama , Haruka Kusakabe , Chien-Hsiu Lee , Michael Maseda , Jorryt Matthee , Leo Michel-Dansac (1) , Tohru Nagao , Themiya Nanayakkara , Moka Nishigaki , Masato Onodera , Yuma Sugahara , Yi Xu

1 CRAL - Centre de Recherche Astrophysique de Lyon

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.

Mots clés: 595; 622; 1569; 580; 416; Astrophysics - Astrophysics of Galaxies

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Yuki Isobe, Masami Ouchi, Akihiro Suzuki, Takashi J. Moriya, Kimihiko Nakajima, et al.. 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. The Astrophysical Journal, 2022, 925, ⟨10.3847/1538-4357/ac3509⟩. ⟨insu-03711537⟩