Trading Oxygen for Iron: Rethinking How the Universe Built Its Stars
The paper by Chruślińska et al. examines how two key elements, oxygen (O) and iron (Fe), trace galaxy growth differently across cosmic time. While astronomers usually measure oxygen abundance, iron behaves differently because it is produced on longer timescales. The authors develop a new framework using the [O/Fe]–specific star formation rate (sSFR) relation to link oxygen-based and iron-based cosmic star formation histories.
Framework and Ingredients
To build oxygen- and iron-dependent metallicity-dependent cosmic star formation histories, the authors assemble several components: the [O/Fe]–sSFR relation, the distribution of galaxy sSFR, the fundamental metallicity relation (FMR) connecting ZO/H and star formation, and the galaxy stellar mass function (GSMF). They update earlier models using new JWST metallicity measurements, which show that galaxies at z > 3 are not as metal-poor as previously assumed. This update directly affects how oxygen and iron abundances evolve over time.
The [O/Fe]–sSFR Relation
A key ingredient is the recently established [O/Fe]–sSFR relation, which links abundance ratios to galaxy star formation activity. The authors explore three enrichment scenarios, “fast,” “slow,” and “mixed”, that bracket current observational uncertainties. These determine how quickly [O/Fe] declines as galaxies age. Using these relations, the model assigns each galaxy both oxygen and iron abundances at all redshifts, enabling the computation of the first full iron-dependent fSFR(ZFe/H, z).
Key Results
The iron-dependent and oxygen-dependent cosmic star formation histories diverge significantly. Iron-based metallicities remain lower because iron enrichment lags behind oxygen. The offset increases toward early times and stabilizes around z ≈ 3. Importantly, solar-like oxygen-to-iron ratios are uncommon: at least 70% of all stars formed at non-solar O/Fe. Most stellar evolution models assume solar abundance patterns, so this result highlights a need for major updates in modeling high-redshift and low-metallicity environments.
Observational Checks
The model matches several observational probes. RR Lyrae stars in dwarf galaxies align with the low-metallicity tail of the model at z ≳ 2–5. Milky Way globular clusters fall within high-SFRD regions expected by the framework. Long gamma-ray burst (LGRB) hosts provide rare gas-phase iron measurements at high redshift, and their dust-corrected [Fe/H] generally agrees with the model’s SFRD-weighted ⟨[Fe/H]⟩. These comparisons show that the iron-based cosmic star formation history behaves realistically.
Model Variations and Low-Metallicity Star Formation
The authors test how assumptions about the galaxy main sequence, GSMF, and high-redshift metallicity evolution affect the total cosmic star formation history. Their variants remain consistent with literature UV and IR measurements. The largest differences appear in the low-metallicity cosmic star formation history, where iron- and oxygen-based estimates diverge sharply. This matters for predictions involving black hole mergers, GRBs, and other low-metallicity transients.
Conclusions and Implications
The authors conclude that non-solar abundance ratios dominate cosmic star formation. While the Sun formed in a roughly typical galaxy at z ≈ 0.4, achieving solar-like O/Fe required a particular enrichment history. Across cosmic time, most stars form with oxygen-enhanced, iron-poor abundance patterns. The study emphasizes that stellar population models and galaxy simulations must treat oxygen and iron separately, and the authors’ framework provides a path toward more realistic modeling of chemical evolution and the cosmic star formation history.
Source: Chruślińska
