Correlations between POL-2 polarisation parameters p_mas and S and thecolumn density. The quantities are calculated at four spatial resolutions represented with the colours indicated in panel a. The data are sampled at steps of FWHM/2 and the solid lines correspond to running averages. In panels a–d, the shading corresponds to N(H₂)< 2 × 10²² cm⁻² where, according to Fig. 9, the 40′′ resolution p_mas estimates become unreliable. In panel d, values for 10²² cm⁻² < N(H₂) < 2 × 10²² cm⁻² and N(H₂) > 2 × 10²² cm⁻² are shown with small and large symbols, respectively, and the lines show running averages for the higher column density interval (20 logarithmic bins over the parameter range with a minimum of four points per bin). Shaded cyan region corresponds to the interquartile range of the quantities plotted with red lines. In panel d the upper solid black line shows the relation log₁₀ (S) = − 0.834 × log₁₀(p_mas) − 0.504 from Planck Collaboration Int. XIX (2015) and the lower black line the relation S = 0.1∕p_mas.