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 deg 2 ) Subaru imaging data in EMPRESS survey. Nine out are EMPGs with an oxygen abundance (O/H) less than ?10% solar value ? , four sources contaminants moderately metal-rich or no emission-line objects. Notably, two nine have low stellar masses abundances 5 × 10 4 –7 M 2%–3% respectively. With sample five (Fe/O) measurements, (three) which taken from this study (the literature), we confirm that lowest ratios ?2% show high ?0.1, close ratio. Comparing galaxy chemical enrichment models, find cannot be explained scenario gas accretion/episodic star formation history due their (N/O) ratios. conclude can reproduced inclusion bright hypernovae and/or hypothetical pair-instability supernovae (SNe) preferentially produced environment. This conclusion implies primordial at z ? could Fe did not originate Type Ia SNe delays may serve as cosmic clock galaxies.