Distribution of β_mm as a function of τ₃₅₃, for the whole region, when the SMICA CMB map is subtracted from each channel map. The black line gives the β_mm–τ₃₅₃ relationship derived from a fit to the points where τ₃₅₃ ≥ 4 × 10^-4, compared to that estimated in Sect. 5 and given by the red line. The colour scale is logarithmic and it represents the density of points. The three contours show the densities for a cumulated fraction of 5, 30, and 56% of the data points, from red to yellow.