Radial density distribution of the oxygen abundance after normalising the abundances of each galaxy so that the abundance at 1r_e is equal to the average value for the whole sample at such radius. The white dots represent the mean oxygen abundance values, with the error bars indicating the corresponding standard deviations, for bins of 0.3 r_e. The solid red line represents the error-weighted linear fit derived for those mean values within the range between 0.5 and 1.5 r_e, and the dashed white line represents the linear relation corresponding to the characteristic value of the slope derived in Sect. 4.2 for the individual galaxies assuming a Gaussian distribution.