Ratio of the ƒ_CO obtained when using a ζ_H ∝χ² without and with an upper limit on ζ_H, as a function of ζ_H in the post-processed galaxy. Here we only show the grid cells with ζ_H greater than the imposed upper limit of 3 × 10⁻¹⁴ s⁻¹ H⁻¹, since the two ƒ_CO are identical for lower CRIR values. Each grid cell is colour-coded by the mean hydrogen density 〈n_H〉 in the cell. The red line shows the mass-weighted mean of the cells in each ζ_H bin. We also indicate by a blue arrow the highest observed ζ_H of 3 × 10⁻¹² s⁻¹ H⁻¹ (from Yang etal. 2023).