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Abstract 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 atM_*∼ 10⁸⁻¹⁰M_⊙along with two quiescent galaxies atM_*∼ 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)_gmeasured using the strong-line method, we are able to make the first-ever apples-to-apples comparison ofαelements in the stars and the interstellar medium. We find that the [Mg/H]_*–[O/H]_grelation is much tighter than the [Fe/H]_*–[O/H]_grelation, which can be explained by the similar production processes ofαelements. Most galaxies in our sample exhibit higher [O/H]_gthan [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]_gcompared to stellar abundances as well as the anticorrelation between sSFR and stellar abundance suggests that galaxy quenching of intermediate-mass galaxies atM_*∼ 10⁸⁻¹⁰M_⊙is primarily driven by starvation. 

Abstract Nyx is a nearby, prograde, and high-eccentricity stellar stream physically contained in the thick disk, but its origin is unknown. Nyx could be the remnant of a disrupted dwarf galaxy, in which case the associated dark matter substructure could affect terrestrial dark matter direct-detection experiments. Alternatively, Nyx could be a signature of the Milky Way’s disk formation and evolution. To determine the origin of Nyx, we obtained high-resolution spectroscopy of 34 Nyx stars using Keck/HIRES and Magellan/MIKE. A differential chemical abundance analysis shows that most Nyx stars reside in a metal-rich ([Fe/H] > −1) high-αcomponent that is chemically indistinguishable from the thick disk. This rules out the originally suggested scenario that Nyx is the remnant of a single massive dwarf galaxy merger. However, we also identify 5 substantially more metal-poor stars ([Fe/H] ∼ −2.0) whose chemical abundances are similar to those of the metal-weak thick disk. It remains unclear how stars that are chemically identical to the thick disk can be on such prograde, high-eccentricity orbits. We suggest two most likely scenarios: that Nyx is the result of an early minor dwarf galaxy merger, or that it is a record of the early spin-up of the Milky Way disk—although neither perfectly reproduces the chemodynamic observations. The most likely formation scenarios suggest that future spectroscopic surveys should find Nyx-like structures outside of the solar neighborhood.