Plot of the Δχ² values of the model fits with different PAH-to-dust mass ratios (M_PAH/M_dust), while the rest of the model parameters are fixed to the best fiducial model, with optimised PAH size range (N_C=10-26) and ionization fraction (ϕ=0.15), as discussed in Sect. 6.2.2, and that best fits the continuum (Sect. 6.1). The spectral regions covering the entire AIB profiles are used for the χ² calculations. The minimum of the plot (Δχ² = 0) corresponds to the minima of the fit through the χ² values (also see text in Sect. 6.2.2). Hence, the light red shaded region with 0 ≤ Δχ² ≤ 1, shows the 1σ confidence limit in the estimation of M_PAH/M_dust, given the fiducial N_C and ϕ values.