We present 3D hydrodynamic simulations aimed at studying the dynamical and chemical evolution of the interstellar medium in dwarf spheroidal galaxies .
This evolution is driven by the explosions of Type II and Type Ia supernovae , whose different contribution is explicitly taken into account in our models .
We compare our results with detailed observations of the Draco galaxy .
We assume star formation histories consisting of a number of instantaneous bursts separated by quiescent periods .
Diverse histories differ by the number of bursts , but all have the same total duration and give rise to the same amount of stars .
Because of the large effectiveness of the radiative losses and the extended dark matter halo , no galactic wind develops , despite the total energy released by the supernovae is much larger than the binding energy of the gas .
This explains why the galaxy is able to form stars for a long period ( > 3 Gyr ) , consistently with observations .
In this picture , the end of the star formation and gas removal must result from external mechanisms , such as ram pressure and/or tidal interaction with the Galaxy .
The stellar [ Fe/H ] distributions found in our models match very well the observed ones .
We find a mean value \langle [ Fe/H ] \rangle = -1.65 with a spread of \sim 1.5 dex .
The chemical properties of the stars derive by the different temporal evolution between Type Ia and Type II supernova rate , and by the different mixing of the metals produced by the two types of supernovae .
We reproduce successfully the observed [ O/Fe ] - [ Fe/H ] diagram .
However , our interpretation of this diagram differs from that generally adopted by previous chemical models .
In fact , we find that the break observed in the diagram is not connected with the onset of a galactic wind or with a characteristic time scale for the sudden switchover of the Type Ia supernovae , as sometimes claimed .
Instead , we find that the chemical properties of the stars derive , besides the different temporal evolution of the SNe II and SNe Ia rates , from the spatial inhomogeneous chemical enrichment due to the different dynamical behaviour between the remnants of the two types of supernovae .