Gerhard Hensler, Univ. of Vienna:

Early evolution of the system of Milky Way satellites in LCDM 
universe

Abstract:
The dwarf spheroidal galaxies (dSphs) around the Milky Way 
(MWG) suffer to belong to the most poorly understood objects 
of astrophysical research and serve as the most challenging 
targets for various reasons:
At first, in LCDM cosmology, a huge number of subhalos are 
expected to surround massive galaxies in contradiction to the 
observed number of satellite galaxies around the MWG. The 
accretion of satellite galaxies by the MWG should have lead to 
the built-up of the Galactic halo and should have left behind 
kinematic and chemical witnesses as stellar streams and 
abundance similarities to the existing dSphs. Both envisaged 
signatures are not observed, however. Although it must be 
expected that the evolution of the baryonic matter in the CDM 
subhalos is heavily influenced by the complex environmental 
processes, yet only evolutionary models of isolated dSphs are 
simulated.
For reliability, not only the effects of star formation and its 
self-regulation, the chemical enrichment by stars and the 
chemo-dynamics of the gas phases are necessary to be 
included, but also tidal effects of the mature galaxy and the 
mutual interactions among the subhalo system itself.
We have, therefore, simulated the evolution of 240 subhalos 
simultaneously around a MWG-type galaxy from the Via Lactea 
II cosmological simulations, starting z=4.56 with their initial gas 
content of 17% of the DM mass. A first snapshot after 1 Gyr 
demonstrates the action of tidal and merging effects on the 
star-formation history, the baryonic and DM mass distribution, 
chemical abundances, but also the formation of galactic halo 
gas, ram-pressure and tidal gas stripping from the dSphs.
As most important results we trace the star formation, 
chemistry and mass distribution within the whole system and 
found that 1) an initial epoch of star formation is more effective 
in lighter subhalos than in more massive ones, 2) gas is more 
easily exhausted from the lower mass units, but 3) can be 
accreted or accumulated by merging of dSphs and lead to later
star-formation events, 4) a low O/Fe agrees with present-day 
dSphs but not with the MWG halo stars, 5) metal-enriched gas 
accumulates and forms the MWG halo, 6) further more.
Our models can however not produce an invisible baryon-free 
subhalo population.