Effects of the galactic winds on the stellar metallicity distribution of dwarf spheroidal galaxies

Aims. To study the effects of galactic winds on the stellar metallicity distributions and on the evolution of Draco and Ursa Minor dwarf spheroidal galaxies (dSphs), we compared the predictions of several chemical evolution models, adopting different prescriptions for the galactic winds (including a model with no wind), with the photometrically-derived stellar metallicity distributions (SMDs) of both galaxies. Methods. We adopted chemical evolution models for Draco and Ursa Minor, which are able to reproduce several observational features of these two galaxies, such as the [alpha/Fe], [Eu/Fe], [Ba/Fe] ratios and the present gas mass. The models take up-to-date nucleosynthesis into account for intermediate-mass stars and supernovae of both types, as well as the effect of these objects on the energetics of the systems. The predictions were compared to the photometric SMDs, which are accurate enough for a global comparison with general aspects such as metallicity range, shape, position of the peak, and high-metallicity tail, leaving aside minor details of the distributions. Results. For both galaxies, the model that best fits the data contains an intense continuous galactic wind, occurring at a rate proportional to the star formation rate. Models with a wind rate assumed to be proportional only to the supernova rate also reproduce the observed SMD, but do not match the gas mass, whereas the models with no galactic winds fail to reproduce the observed SMDs. In the case of Ursa Minor, the same model as in previous works reproduces the observed distribution very well with no need to modify the main parameters of the model (star formation efficiency nu = 0.1 Gyr(-1) and wind efficiency w(i) = 10). The model for Draco, on the other hand, is slightly modified. The observed SMD requires a model with a lower supernova type Ia thermalization efficiency (eta(SNeIa) = 0.5 instead of eta(SNeIa) = 1.0, as used in previous papers of this series) in order to delay the galactic wind, whereas all the other parameters are kept the same (nu = 0.05 Gyr(-1), w(i) = 4). Conclusions. The model results, compared to observations, strongly suggest that intense and continuous galactic winds play a very important role in the evolution of local dSphs. They not only remove a large fraction of the gas content of the galaxy, but also define the pattern of the abundance ratios and of the SMD, giving rise to low (below solar) values for [alpha/Fe], [Eu/Fe], as well as high [Ba/Fe] at high metallicities, as observed, and preventing metal-rich stars being formed, which is reflected in the strong decrease in the high-metallicity tail of the stellar distributions in these galaxies.