Star formation feedback and metal enrichment by Types Ia and II supernovae in dwarf spheroidal galaxies: the case of Draco

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 supernovae (SNe II) and Type Ia supernovae (SNe Ia), 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 [Fe/H]=−1.65 with a spread of ∼1.5 dex. The chemical properties of the stars derive by the different temporal evolution between SNe Ia and SNe II 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 SNe Ia, 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.