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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

Abstract : 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.
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Submitted on : Friday, July 1, 2022 - 3:22:10 PM
Last modification on : Saturday, September 24, 2022 - 12:02:05 PM

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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⟩

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