Probability density functions of the molecular hydrogen column density derived from Hi-GAL (Fig. 2). The dashed vertical line marks the column density threshold, and the solid vertical line marks the mean column density. We list in the figure the mean column density ⟨N(H₂)⟩, the log-normal (green curve) width σ_LN, and the power-law index s (solid line). Error bars are calculated from Poisson statistics.