7. WHAT HAVE WE LEARNED ABOUT dSphs?

From the study of the chemical evolution of dSphs and the Milky Way we can derive the following conclusions:

• By comparing the [/Fe] ratios in the Milky Way and in dSphs of the Local Group we can conclude that these systems had different histories of SF.

• The [/Fe] ratios in dSphs are always lower than in the Milky Way at the same [Fe/H]. This is a consequence of the time-delay model which predicts this behaviour for systems which suffered a lower star formation than the solar vicinity.

• The occurrence of strong galactic winds or gas loss in general is necessary to keep the SF low and it produces the steep decrease of the [/Fe] ratio observed in dSphs (see Lanfranchi & Matteucci 2007).

• Good agreement is found both for [s/Fe] and [r/Fe] abundance ratios. The [s/Fe] ratios are predicted to be higher than the same ratios in Milky Way stars with the same [Fe/H]. This is again a consequence of the time-delay model.

• The dSphs of the Local Group contain very old stars but they suffered extended periods of SF, far beyond the reionization epoch. This is suggested both from the color-magnitude diagrams of these galaxies and from the level of the abundances of s-process elements such as Ba, which could not have been observed if the SF had stopped at the reionization epoch (Fenner et al. 2006).

• All the previous conclusions suggest that it is unlikely that the dSphs have been the building blocks of the Milky Way, as predicted by current CDM models (see review by Geisler et al. 2007 for a detailed discussion of this point). Robertson et al. (2005) studied the formation of the Galactic stellar halo by means of different accretion histories for the dark matter halo of the Milky Way in the framework of the CDM model. They concluded, on the basis of the [/Fe] ratios in Galactic halo stars, in dwarf irregulars and dSphs, that it is more likely that the Galactic dark matter halo was formed by an early accretion of dwarf irregular galaxies, which formed stars for a short time and then were destroyed. Concerning dSphs, they suggest that their chemical abundances should have been affected by galactic winds and that they should have been accreted and destroyed over the entire Milky Way lifetime.