CO velocity-integrated T_BI_CO plotted versus (logarithm) visual extinction A_V (left panel) and (logarithm) visual dust optical depth τ_V (right). The top axes show the scale for τ₁₆₀ assuming that A_V/τ₁₆₀ = 2180, as described in Sect. 5.3. The light curves show fits to Eq. (12). Heavy curves show the medians for each galaxy binned in log₁₀(A_V), (left), log₁₀(τ_V), (right), with the shaded regions corresponding to ±1σ deviations. Also shown in both panels are the MW GMCs studied by Lee et al. (2018), with A_V inferred from Planck; see Sect. 5 for more details.