Radial profile in observed angular units of Hβ surface brightness normalised to the maximum in the three defined regions: red diamonds for region A, blue squares that is for region B, and green crosses for region C. The coloured solid lines represent the predictions made by the models in the MB thin-shell approach, while the black dashed line represents the prediction of the model in the RB thick-shell approach (see Sect. 3 for a explanation of the models).