of semi-analytical models, in particular dealing with the calculation of the infrared emission of early-type galaxies. Second, we will discuss how dynamical simulations and a dusty spectrophotometric code, when mixed together allow to move a step forward in the calculations of the SEDs properties. Third, we will show in detail how our model has been built and the coupling between dynamics and dusty population synthesis has been done.

The semi-analytical models and their drawbacks
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The semi-analytical models approximate a galaxy to a point mass system in which gas is turned into stars by means of suitable recipes for star formation and heavy elements are produced by stellar nucleosynthesis and stellar winds/explosions. The standard evolutionary population synthesis technique (EPS) is usually applied to derive the SED of the galaxy, with models able to explain many global features of early type galaxies, as amply described by many authors [@Arimoto87; @Arimoto89; @Bressan94; @Gibson97b; @Tantalo96; @Tantalo98]. There are three important and problematic issues of these models to be discussed for our purposes.

First, to determine the age at which the galactic wind sets [@Larson74; @Larson75], we need some hypothesis about Dark and Baryonic Matter with their relative distributions, and about the heating and 