  The Via Lactea simulation of the dark matter halo of the Milky Way predicts
the existence of many thousands of bound subhalos distributed approximately
with equal mass per decade of mass. Here we show that: a) a similar steeply
rising subhalo mass function is also present at redshift 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 two
more subhalos with large circular velocities; b) the fraction of Via Lactea
mass brought in by subhalos that have a surviving bound remnant today with
present-day peak circular velocity Vmax>2 km/s (>10 km/s) is 45% (30%); c)
because of tidal mass loss, the number of subhalos surviving today that reached
a peak circular velocity of >10 km/s throughout their lifetime exceeds half a
thousand, five times larger than their present-day abundance and more than
twenty times larger than the number of known satellites of the Milky Way; e)
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 today must be
housing a luminous dwarf; f) small dark matter clumps appear to be relatively
inefficient at forming stars even well beyond the virial radius; g) 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; h) subhalos have central densities that
increase with Vmax and reach 0.1-0.3 Msun/pc3 comparable to the central
densities inferred in dwarf spheroidals with core radii >250 pc.
