Probing Chemical Enrichment in Extremely Metal-poor Galaxies

Abstract

The chemical composition of galaxies offers vital insights into their formation and evolution. In particular, the relationship between helium abundance (He/H) and metallicity serves as a key diagnostic for estimating the primordial helium yield from Big Bang nucleosynthesis. We investigate the chemical enrichment history of low-metallicity galaxies, focusing especially on extremely metal-poor galaxies (EMPGs), using one-zone chemical evolution models. Adopting elemental yields from M. Limongi & A. Chieffi, our models reach He/H ∼ 0.089 at (O/H) × 105 < 20, yet they fall short of reproducing the elevated He/H values observed in low-redshift dwarf galaxies. In contrast, the observed Fe/O ratios in EMPGs are successfully reproduced using both the K. Nomoto et al. and M. Limongi & A. Chieffi yield sets. To address the helium discrepancy, we incorporate supermassive stars (SMSs) as Population III stars in our models. We find that SMSs can significantly enhance He/H, depending on the mass-loss prescription. When only 10% of the SMS mass is ejected, the model yields the steepest slope in the (O/H) × 105–He/H relation. Alternatively, if the entire outer envelope up to the CO core is expelled, the model can reproduce the high He/H ratios observed in high-redshift galaxies (He/H > 0.1). Additionally, these SMS-enriched models also predict elevated N/O ratios, in agreement with recent JWST observations of the early Universe.