dusty population synthesis models
--------------------------------------------------------------------

The description of an early-type galaxy as far as predicting its spectro-photometric infrared properties can be therefore realized with a suitable combination of dynamical and spectro-photometric approaches. Coupling the dynamical models with spectro-photometric synthesis requires a number of steps that deserve some remarks.

### Radial density profiles.

Fig. \[RaggiViriali\] shows the cumulative distribution of gas and stars as a function of the radial galactocentric distance normalized to the virial radius for model A (top panel) and model B (bottom panel). The gas is generally distributed in the external regions of the galaxy and steeply decreases inward. In contrast the stars are more concentrated toward the centre. The gradients in the spherically averaged star- and gas- content provided by the dynamical models are the primary information to load into the spectro-photometric code of @Piovan06b. They allow us to infer the amount of gas contained within a given radius or within a given aperture. We fix the total dimension of portion of the average model producing the IR flux at a diameter $D_{gal} = 25$ kpc, consistent with the mean galaxy size of the observed sample.

As the spectro-photometric code of @Piovan06b suited to describe early-type galaxies 