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

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 (similar to 26 AB mag) optical and wide-area (similar to 500 deg(2)) Subaru imaging data in the EMPRESS survey. Nine out of the 13 candidates are EMPGs with an oxygen abundance (O/H) less than similar to 10% solar value (O/H)(circle dot), 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 x 10(4)-7 x 10(5) M (circle dot) and 2%-3% (O/H)(circle dot), 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 similar to 2% (O/H)(circle dot) show high (Fe/O) ratios of similar to 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 similar to 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.

Automated summary

This study conducted Keck/LRIS follow-up spectroscopy for 13 photometric candidates of extremely metal-poor galaxies, confirming that two EMPGs with the lowest oxygen ratios show high iron ratios close to solar abundance. The inclusion of bright hypernovae and/or pair-instability supernovae in metal-poor environments could explain the characteristics of these galaxies.