"The Tumultuous Lives of Galactic Dwarfs and the Missing Satellites Problem"

Hierarchical Cold Dark Matter (CDM) models predict that Milky Way (MW) sized halos contain hundreds of dense low-mass dark satellites, an order of magnitude more than the number of observed satellites in the Local Group (LG). If the CDM paradigm is correct, we need to understand why most of these halos failed to form stars and become galaxies (or, put differently, why only 10% did?). I will present a simple solution to this "missing satellites problem" using results of high-resolution cosmological simulation of MW sized halos in the LCDM cosmology. I will show that about 10% of the substructure halos with the present masses <108-109 Msun, similar to the observed masses of dwarf spheroidal galaxies, had considerably larger masses and circular velocities when they formed at redshifts z>2. After the initial period of mass accretion in isolation, these objects experience dramatic mass loss due to tidal stripping, in some cases even before they are accreted by their host halo. This can explain how the smallest dwarf spheroidal galaxies of the LG were able to build up a sizable stellar mass in their seemingly shallow potential wells. I will show that with a couple of reasonable assumptions our results can explain the abundance, spatial distribution, and morphological segregation of the observed Galactic satellites.