EMPRESS. IV. Extremely Metal-Poor Galaxies (EMPGs) Including Very Low-Mass Primordial Systems with M*=10^4--10^5 M_sun and 2--3% (O/H)_sun: 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 (∼ 26 AB mag) optical and wide-area (∼ 500 deg) 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 5×10–7×10 M and 2–3% (O/H) , respectively. Examining a sample of 5 EMPGs with (Fe/O) measurements, two (three) of which are taken from this study (the literature), we confirm that very young EMPGs with Hβ ar X iv :2 10 8. 03 85 0v 1 [ as tr oph .G A ] 9 A ug 2 02 1