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 Limongi and Chieffi (2018), our models reach He/H ~ 0.089 at (O/H) $\times 10^5<20$, 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 Nomoto et al. (2013) and Limongi and Chieffi (2018) yield sets. To address the helium discrepancy, we incorporate supermassive stars (SMSs) as Pop III stars in our models. We find that SMSs can significantly enhance He/H, depending on the mass-loss prescription. When only 10 percent of the SMS mass is ejected, the model yields the steepest slope in the (O/H) $\times 10^5$ - 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.