Metals in Star-forming Galaxies with KCWI. I. Methodology and First Results on the Abundances of Iron, Magnesium, and Oxygen

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 8−10 M ⊙ along with two quiescent galaxies at M * ∼ 10 8.8 M ⊙ 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 8−10 M ⊙ is primarily driven by starvation.