Dark Matter Subhalos and the Dwarf Satellites of the Milky Way

The Via Lactea simulation of the cold dark matter halo of the Milky Way predicts the existence of many thousands of bound subhalos with masses above 106 M⊙, distributed approximately with equal mass per decade of mass. Here we show that (1) a similar steeply rising subhalo mass function is also present atredshiftz = 0.5 in an elliptical-sized halo simulated with comparable resolution in a different cosmology. Compared to Via Lactea, this run produces nearly a factor of 2 more subhalos with large circular velocities; (2) the fraction of Via Lactea mass brought in by subhalos that have a surviving bound remnant today reaches 45%. Most of the Via Lactea mass is acquired in resolved discrete clumps, with no evidence for a significant smooth infall; (3) because of tidal mass loss, the number of subhalos surviving today that reached a peak circular velocity of > 10 km s-1 throughout their lifetime exceeds half a thousand, 5 times larger than their present-day abundance; (4) unless the circular velocity profiles of Galactic satellites peak at values significantly higher that expected from the stellar line-of-sight velocity dispersion, only about one in five subhalos with Vmax > 20 km s-1 today must be housing a luminous dwarf; (5) nearly 600 halos with masses greater than 107M⊙ are found today in the "field" between r200 and 1.5r200, i-e. small dark matter clumps appear to be relatively inefficient at forming stars even well beyond the virial radius; 6) the observed Milky Way satellites appear to follow the overall dark matter distribution of Via Lactea, while the largest simulated subhalos today are found preferentially at larger radii; (7) subhalos have central densities that increase with Vmax and reach ρDM = 0.1-0.3 M⊙ pc-3, comparable to the central densities inferred in dwarf spheroidals with core radii >250 pc.