model, morphology can be defined in terms of the galaxy physical parameters, like the mass of the bulge and the disk (see Tonini et al. 2011). This method has the advantage of grouping together objects that share a similar formation history, thus favouring a more detailed study of the physics involved in their evolution. On the other hand, this type of selection is hard to apply to observations, and it involves some model-dependencies in the conversion between colors and luminosities to masses and ages, thus confusing the comparison between models and data.

![The distribution of the mass ratio $M_{\rm bulge}/M_{\rm disk}$ for the model Sc, Sb and Sa galaxies (*blue, orange and red respectively*).[]{data-label="histo"}](f1.eps)

In most observations the determination of the Hubble type employs the use of some photometric criterion, based on colors or on the relative luminosities of bulge and disk when available. To facilitate the comparison, we classify our model galaxies with one of such methods, based on the bulge-to-total luminosity ratio in the B band, following Simien $\&$ De Vaucouleurs (1986): after defining $\mu_B=M_B(bulge)-M_B(total)$, Sc galaxies are characterised by $2.3 < \mu_B < 4.15$, Sb galaxies by $1.23 < \mu_B < 2.01$ and Sa galaxies by $0.8 