Effect of individual carbon and oxygen abundances on CO predictions

Determine how using the individual carbon and oxygen abundances tracked by the FIRE-2 simulations, rather than scaling all elemental abundances solely with total metal mass fraction, changes the predicted CO abundances and CO luminosities of simulated galaxies.

Background

The pipeline scales CLOUDY elemental abundances using each gas cell’s total metallicity and does not use the separately tracked carbon and oxygen abundances. Since CO formation depends directly on carbon and oxygen abundances, gas cells with identical total metallicity but different C/H or O/H ratios could produce different CO abundances and luminosities.

The authors explicitly state that this effect has not been tested and identify extending the CLOUDY lookup table to include separate carbon and oxygen abundances as the required next step.

References

Gas with the same total metallicity but a different C/H or O/H ratio may therefore produce a different CO abundance and luminosity than we predict. Such differences may be systematic with the star formation history, assumed initial mass function and the redshift of the galaxy. We have not tested this effect. Doing so would require extending the CLOUDY table with separate carbon and oxygen abundances, and we leave it for future work.

— Modeling CO Line Luminosity Using FIRE Simulations: I. Methodology and Validation of Molecular Gas and CO Abundance  (2610.12159 - Tolgay et al., 8 Oct 2026) in Section 8, “Caveats”