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^2) 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)_sun, 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*10^4--7*10^5 M_sun and 2--3% (O/H)_sun,
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)_sun 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 an 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 is not
originated from Type Ia SNe with delays, and that Fe may not serve as a cosmic
clock for primordial galaxies.
