is written in spherical symmetry, we have to derive suitable spherical distributions for the density of stars and gas to be used into the model. The task is facilitated by the nearly spherical shape of the dynamical models. To this aim, we consider the sphere of radius $R_{gal}$ centred at the centre of mass of the stellar component. The sphere is then divided in a number of thin spherical shells whose derived average density of stars and gas is shown in Fig. \[Fit\_RHO\]. Even if the centre of mass of the star and gas distributions may not be exactly coincident, this not relevant here, so that the same coordinate centre can be used for both components.

In order to secure a smooth behaviour at the galaxy radius $R_{gal}$ the star and gas density profiles are represented by the law:

$$\rho _{i}=\rho _{0i}\left[ 1+\left( \frac{r}{r_{c}^{i}}\right)
^{2}\right] ^{-\gamma _{i}}  \label{rhostar_ell}$$

where $``i"$ stands for $``stars"$ or $``gas "$, $r_{c}^{i}$ are the corresponding core radii. The above representation is more suited to our aims than the classical King law. The fits are shown in Fig. \[Fit\_RHO\] (solid lines). They are normalized in such a way that the integral over the galaxy volume corresponds 