Comparison of observed and modelled (J13, W = 1) R-band surface brightness. Frame a: map of the ratio with contours at equal to 20 kJy sr⁻¹ and 40 kJy sr⁻¹ (cf. Fig. 7a). The red circle indicates the reference area used for establishing a common zero point. Frame b: correlation as 2d histogram, with a logarithmic colour scale for the point density. stands for observations corrected for the main gradient. The dashed line shows the least-squares fit to data with kJy sr⁻¹. The effect of background sky brightness is not included in the model.