interval and (2) the star formation history that is reversed allowing longer SFH for more massive galaxies. These two problems, combined with a lack of geometrical information about the distribution of gas and dust, make semi-analytical models not suitable to calculate properly the IR emission of early type galaxies. To improve upon them we need to use the results obtained from dynamical simulations. They have shown to be able to properly model the ejection of gas by the galaxy as a sort of continuous process, taking place whenever a gas particle heated up by various mechanism has acquired a velocity greater than the escape velocity [see e.g. @Carraro98; @Kawata01; @Springel01; @Chiosi02]. They are able to reproduce the SF history of early-type galaxies both in the context of the monolithic collapse scenario [@Kawata01; @Chiosi02] and recently in the context of hierarchical scenario [@DeLucia06].

Finally, the galaxy is no more a mass point, but a fully three-dimensional structure of the galaxy is available with spatial distribution of stars and gas.

@Merlin06, with the aid of *N-Body Tree-SPH* simulations based on quasi-cosmological initial conditions in the standard-Cold Dark Matter scenario (S-CDM), modelled the formation and evolution of two early-type galaxies of different total 