Observed and predicted radial gas surface density gradient. The black dashed curve is the average between the Dame (1993) and Nakanishi & Sofue (2003, 2006) data sets. The grey shaded region represents the typical uncertainty at each radius, for which we adopt either 50% of the average (see Nakanishi & Sofue 2006) or half the difference between the minimum and maximum values in each radial bin (if larger). The big filled circles show the model predictions.