H₂ surface density derived from CO and K_dark derived from the Bayesian analysis. The continuous curve shows Σ_H2 based on Fig. 10 of Druard et al. (2014) corrected to a X_CO factor of 1.1 Galactic and uncorrected for inclination and helium content. The histograms show the CO dark gas surface density. The solid histogram shows K_dark as derived assuming that all positions have the same dark column as the positions where CO is detected above the threshold. The dashed and dotted histograms represent K_dark assuming that the dark column only is present where CO is detected above the 0σ and 3σ thresholds respectively.