Galaxy formation and dark-matter substructure

In the current paradigm of cosmic structure formation (ACDM), galaxies like the Milky Way are embedded within extended halos of dark matter, which form through the gravit at ionally induced aggregation of many smaller progenitors. The merging of these fragments is rather inefficient, and leaves substantial amounts of "substructure" orbiting the host. Despite heavy-mass loss the largest subunits are expected to survive tidal disruption and host luminous dwarf satellites. Smaller clumps are left nearly unscathed, appear to have no optically luminous counterparts, and their signature may be detectable in gravitationally lensed images of background quasars or via γ-rays from dark-matter annihilation in their cores. To compare the rich mass distribution predicted by this model with the recent explosion of data probing galactic substructure and the accretion history of the Milky Way requires detailed calculations of the highly nonlinear dynamics of hierarchical gravitational growth. In this paper I will discuss results from the largest cosmological simulations to date of the assembly of the galactic cold dark-matter halo.