CO conversion factor versus gas-to-H₂ ratio at epochs 30 Myr (top panel) and 100 Myr (bottom panel). The solid lines correspond to calculations of α as a function of the gas-to-H₂ ratio η_g,H2 for a CO line luminosity of  K km s^-1 pc². Calculations are performed for different IMFs and galaxies for a range of different initial gas masses M_ini. The thickness of the lines is scaled by M_ini as indicated in the upper panel. The black and cyan colors denote the Salpeter and top-heavy IMF, respectively. The arrow indicates the shift of α for calculations with the lower  K km s^-1 pc², and Δ_α is the difference of α between the higher and lower . Calculations are shown for models with ψ_ini = 3 × 10³ M_⊙ yr^-1, except for the model for the most massive galaxy for which ψ_ini = 10⁴ M_⊙ yr^-1. The grey shaded region signifies the possible range of α and η_g,H2. The horizontal black dashed lines mark the values of α = 0.8, 1 and 4.6 M_⊙ (K km s^-1 pc²)^-1.