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.
[/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
- 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
- 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
- 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
for a detailed discussion of this point).
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.