Dark matter substructure within galactic halos

We use numerical simulations to examine the substructure within galactic an d cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches the observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarka ble result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialized extent of the Milky Way's halo shoul d contain about 500 satellites with circular velocities larger than the Dra co and Ursa Minor systems, i.e., bound masses greater than or similar to 10 (8) M. and tidally limited sizes greater than or similar to 1 kpc. The subs tructure clumps are on orbits that take a large fraction of them through th e stellar disk, leading to significant resonant and impulsive heating. Thei r abundance and singular density profiles have important implications for t he existence of old thin disks, cold stellar streams, gravitational lensing , and indirect/direct detection experiments.