Error bars including photon noise and sky level uncertainty have been taken into account in the plot. Note in the figure the blue colour bump about from the centre due to the knotty ring. A smooth colour gradient is also evident along galaxy radius with the outermost regions sensibly bluer than the centre. As expected, N50 colours are slightly bluer than high-mass ellipticals, and intermediate between E and Sa Hubble types. A more detailed match of the population synthesis predictions with the N50 colour profile and with the nuclear knotty features is attempted in the insert panel of Fig. 4 .
Colours become bluer at larger distance.
A substantial agreement seems to exist between theoretical models and observations within the zero-point uncertainty in the magnitude scale. Data in Fig. In addition, one should also consider a little blue shift of all the galaxy data by and to take into account for k-correction. The spread in age among the galaxy stellar population is even more evident from the analysis of the integrated spectrum, shown in Fig. 5 . Absolute flux calibration reproduces galaxy energy distribution within the central aperture.