Study Shows Most Cosmic Star Formation Occurred at Non-Solar Oxygen-to-Iron Ratios
Researchers have developed an observationally-motivated framework to independently track how oxygen and iron abundances evolved across cosmic history, finding that at least 70% of all stellar mass formed under non-solar oxygen-to-iron ratios. Because oxygen and iron originate from different nucleosynthetic processes and operate on different timescales, galaxy formation models that treat them as a single metallicity proxy may systematically misrepresent physical conditions. The findings have direct implications for predicting rates of gravitational-wave events and other transients tied to metal-poor stellar progenitors.
A study accepted by Astronomy & Astrophysics introduces a framework for deriving separate oxygen- and iron-dependent cosmic star formation histories (cSFH) by applying an empirical relationship between alpha-element enhancement and galaxies' specific star formation rates. The authors find that star formation occurring at near-solar oxygen-to-iron ratios is rare, with at least 70% of integrated cosmic stellar mass assembling under non-solar O/Fe conditions. The cosmic average metallicity expressed in iron ([Fe/H]) is consistently lower than when expressed in oxygen ([O/H]) by up to a factor of three, with this offset growing from redshift z=0 to z~3 before converging toward core-collapse supernova O/Fe ratios at higher redshifts. The framework is validated against independent observational probes, including iron abundances inferred from absorption spectra of long gamma-ray burst host environments. Because oxygen primarily governs gas cooling rates while iron controls stellar atmosphere opacities and thus stellar evolution and feedback, conflating the two elements introduces systematic errors in galaxy formation simulations. The results carry particular weight for predicting merger rates of compact objects such as binary black holes, whose progenitors are sensitive to iron-based metallicity thresholds rather than the oxygen-based metallicities typically reported from galaxy surveys.
What's missing
The framework relies on the empirical [O/Fe]–sSFR relation from Chruślińska et al. (2024) as its observational anchor; systematic uncertainties or scatter in that underlying relation are a key caveat for the derived cSFH. The framework also lacks direct observational constraints on iron abundance for the bulk of star-forming galaxies at high redshift, where gas-phase oxygen is the only routinely measured metallicity tracer, leaving the iron-dependent cSFH at z > 2 less well constrained.
What different sources said
- arXiv astro-phCenter
Trading oxygen for iron II. Oxygen- versus iron-dependent cosmic star formation history
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