Trading oxygen for iron

Due to their different nucleosynthetic origin, a stellar population produces oxygen (O) and iron (Fe) on different timescales, and their relative abundance can deviate strongly from solar. Galaxy formation models should treat these elements separately, as they play a distinct role in shaping physical phenomena. For example, oxygen mainly sets the gas cooling rate, while the iron abundance sets stellar atmosphere opacities impacting stellar evolution, spectra, and feedback. Observations of star-forming galaxies usually only constrain gas-phase oxygen abundance, vastly limiting our capabilities of separating the cosmic evolution of oxygen and iron. Here, we present an observationally motivated framework to scale the cosmic evolution of O and Fe abundances. We applied the relation between the α -enhancement and galaxies’ specific star formation rate ([O/Fe]–sSFR) to derive the Fe and O-dependent cosmic star-formation history (cSFH). We find that star formation with near-solar O/Fe is rare: at least 70% of the integrated cosmic stellar mass forms at non-solar O/Fe. The cosmic average metallicity is generally lower in [Fe/H] than in [O/H] (by up to a factor 3), with the offset increasing from z = 0 to redshifts z ≈ 3 and then approaching the core-collapse O/Fe ratio. We validated our results against samples that probed the Fe-dependent cSFH in different regimes such as absorption-derived ⟨[Fe/H]⟩ from long gamma-ray bursts. Our results impact the interpretations of stellar and galaxy spectra and the predicted rates of transients, especially those linked to metal-poor progenitors (e.g. black hole mergers).