GLOW. II. A Census of Oxygen in Low-mass Galaxies*

Oxygen is forged by stars and redistributed through galaxies by feedback-driven outflows, leaving a record of star formation and the baryon cycle imprinted on its present-day abundance and distribution. The Galaxies Losing Oxygen via Winds project quantifies the production, distribution, and retention of oxygen in 37 low-mass, low–metallicity, gas-rich galaxies in the nearby Universe ( D < 6 Mpc) spanning a critical stellar mass range (10 6.5 < M * < 10 9.5 ) where feedback is expected to strongly shape galaxy evolution. We incorporate literature results to extend the analysis over 10 5 < M * < 10 11.5 . We couple star formation histories derived from resolved stars with measurements of interstellar medium metallicity and gas mass, and compare the oxygen budgets with empirical constraints in the circumgalactic medium (CGM). We find low-mass galaxies retain between 4% and 25% (mean 11%) of their oxygen produced by stellar nucleosynthesis, and nearly all retained oxygen resides in the gas. Galaxies exhibit increased scatter to higher oxygen retention fractions when residing in denser environments (Θ 5 > 0.5) and within 1 Mpc of a massive galaxy ( M * > 10 9 M ⊙ ), indicating the environment likely affects the amount of oxygen retained, recycled, or accreted to galaxies. Contrary to expectations, oxygen retention does not correlate with position on the mass–metallicity relation. Although ionized oxygen is detected in the CGM, it remains unclear whether most missing oxygen resides there or has been expelled entirely. Hydrodynamical simulations, despite successfully reproducing the mass–metallicity relation, predict much higher retention fractions than measured in low-mass galaxies. Simple modeling indicates that wind mass-loading and outflow metallicity govern oxygen retention, and that low-mass galaxies accrete less baryonic material relative to the cosmic baryon fraction.