Pixel-to-pixel SPIRE_500 μm absolute excess as a function of the column density of the atomic, and detected molecular phase. The spatial resolution is R4, the dust model is “AC”. The color scale is identical to Fig. 13. The error bars correspond to the binned trends. Σ_H2 is the column density of molecular gas derived from the CO line observations, but does not account for the dark component detected in Sect. 4.3. The yellow dashed line shows the zero value (no excess).