Halo Substructure and Disk Heating in a Lambda CDM Universe

We examine recent suggestions that substructure in cold dark matter (CDM) halos may be in conflict with the presence of thin, dynamically fragile stellar disks. N-body simulations of an isolated disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities and orbits derived from high-resolution cosmological CDM simulations indicate that substructure at $z=0$ plays only a minor dynamical role in the heating of the disk over several Gyrs. This is because the orbits of satellites in present-day CDM halos seldom take them near the disk, where their tidal effects are greatest. Unless the effects of substructure are very different at earlier times, our models suggest that substructure might not preclude virialized CDM halos from being acceptable hosts of thin stellar disks like that of the Milky Way.