Example of our DIG modelling and its comparison with the observed Σ_Hα for NGC 4535. Top row: observed Hα flux map (H II regions are masked and appear white in all the maps) and the model predictions for different values of k₀, generated according to Eq. (1). Middle row: a spatial comparison of the model and observations, in the form of log(Model/Data). Bottom row: median relation between log(Model/Data) and Σ_{Hα,  observed} with associated scatter (the shaded region corresponds to the range between the 16th and 84th percentiles). The for each model is also listed above each panel. The model with k₀ ∼ 1 kpc⁻¹ offers a better representation of the data with respect to models with higher (k₀ = 10 kpc⁻¹) or lower values (k₀ = 0 kpc⁻¹, i.e. only spherical dilution).