Probability distribution function of H₂ column density of M 33. The column density was derived from all pixels in the CO map from Fig. 1, excluding only the noisy edges. The left y-axis gives the normalized probability p(η), the right y-axis the number of pixels per log bin. The upper x-axis is in units of H₂ column density, and the lower x-axis is the logarithm of the normalized column density. The green curve indicates the fitted PDF. The dispersion of the fitted PDF is indicated by σ.