over inside the 25 kpc radius region. Similarly in Fig. \[SMALL\_SF\], we show the results obtained inserting the SFH of Model B into a classical chemical model with total baryonic mass of $3.5 \cdot 10^{9} M_{\odot}$. The only difference is that the maximum age of the dynamical model is 5 Gyr. This cross checking of the models is particularly significant because: first, it secures that the results of the analytical models fairly reproduce those of the dynamical simulations as far as some important features are concerned; second, it secures that we can safely use the result of chemical models to prolong the evolutionary history of Model B up to the present; third, that we can safely apply the population synthesis technique of @Piovan06b.

Knowing the amount of gas, we need to specify the fraction of it in form of dust to finally be able to derive the whole SED from X to FIR and look for relationships between the luminosity in the X, B and FIR pass-bands we want to interpret. Our models, both semi-analytical and chemo-dynamical, are not suitable to describe the evolution of the compositions and abundances of *both* gas and dust phases. The relative proportions of the 