2024ApJ...972..182Z

Understanding the chemical enrichment of different elements is crucial to gaining a complete picture of galaxy chemical evolution. In this study, we present a new sample of 46 low-redshift, low-mass star-forming galaxies at M_* ∼ 10^8–10M_⊙ along with two quiescent galaxies at M_* ∼ 10^8.8M_⊙ observed with the Keck Cosmic Web Imager, aiming to investigate the chemical evolution of galaxies in the transition zone between Local Group satellites and massive field galaxies. We develop a novel method to simultaneously determine stellar abundances of iron and magnesium in star-forming galaxies. With the gas-phase oxygen abundance (O/H)_g measured using the strong-line method, we are able to make the first-ever apples-to-apples comparison of α elements in the stars and the interstellar medium. We find that the [Mg/H]_*-[O/H]_g relation is much tighter than the [Fe/H]_*-[O/H]_g relation, which can be explained by the similar production processes of α elements. Most galaxies in our sample exhibit higher [O/H]_g than [Fe/H]_* and [Mg/H]_*. In addition, we construct mass-metallicity relations (MZRs) measured as three different elements (Fe_*, Mg_*, O_g). Compared to the gas O-MZR, the stellar Fe- and Mg-MZRs show larger scatter driven by variations in specific star formation rates (sSFR), with star-forming galaxies exhibiting higher sSFR and lower stellar abundances at fixed mass. The excess of [O/H]_g compared to stellar abundances as well as the anticorrelation between sSFR and stellar abundance suggests that galaxy quenching of intermediate-mass galaxies at M_* ∼ 10^8–10M_⊙ is primarily driven by starvation.