Metallicity dependence of the onset of bright CO emission

Determine why increasing the assumed metallicity of Milky Way molecular clouds by a factor of three shifts the characteristic molecular hydrogen column density for the onset of bright CO emission by approximately the same factor.

Background

The CLOUDY calculations predict that CO luminosity rises sharply once molecular clouds attain sufficient dust shielding, approximately at visual extinction A_V greater than or approximately equal to 1. For a fixed solar metallicity, the rise occurs near N_H2 approximately 3 × 1021 cm-2, whereas for a three-times-solar metallicity it occurs near N_H2 approximately 1021 cm-2.

Because visual extinction scales approximately as A_V proportional to Z N_H, the paper identifies a plausible shielding-based explanation for the shift, but explicitly poses the quantitative reason for the factor-of-three correspondence as an unresolved question.

References

Why does a factor of three increase in metallicity shift the characteristic column density for the onset of bright CO emission by a factor of three?

— 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 3, “Cloudy CO predictions for MW Molecular Clouds”