X_CO estimated from Eq. (14) is plotted as a function of A_V in the left panel, and of τ_V in the right. As in Fig. 8, we include as horizontal dashed lines, X_CO that would be estimated from the metallicity dependence found by Hunt et al. (2020), as X_CO ∝ Z^−1.55. The MW X_CO = 2.3  ×  10²⁰ cm⁻²/K km s⁻¹ is also given together with the uncertainties. Heavy curves show the medians for each galaxy binned in (left) and τ_V (right) with shaded regions showing the 1σ standard deviations.