Map of the 3.4/3.3 μm ratio (color scale) with the contours of the H₂ S(3) line a), and of the ratio I_eVSG/I_PAH = I_eVSG/ (I_PAH⁰ + I_PAH⁺)b). The density plot of the I_3.4/I_3.3 band ratios vs. G₀ is also shown c). In red, we report the linear regression of the data points. Only the pixels for which I_3.3> 1.0 × 10^-7 W m^-2 sr^-1 have been used to build in the analysis of the 3.4/3.3 μm ratio.