Not too big, not too small: the dark halos of the dwarf spheroidals in
  the Milky Way
  We present a new analysis of the Aquarius simulations done in combination
with a semi-analytic galaxy formation model. Our goal is to establish whether
the subhalos present in LCDM simulations of Milky Way-like systems could host
the dwarf spheroidal (dSph) satellites of our Galaxy. Our analysis shows that,
contrary to what has been assumed in most previous work, the mass profiles of
subhalos are generally not well fit by NFW models but that Einasto profiles are
preferred. We find that for shape parameters alpha = 0.2 - 0.5 and Vmax = 10 -
30 km/s there is very good correspondence with the observational constraints
obtained for the nine brightest dSph of the Milky Way. However, to explain the
internal dynamics of these systems as well as the number of objects of a given
circular velocity the total mass of the Milky Way should be ~ 8x10^11 Msun, a
value that is in agreement with many recent determinations, and at the low mass
end of the range explored by the Aquarius simulations. Our simulations show
important scatter in the number of bright satellites, even when the Aquarius
Milky Way-like hosts are scaled to a common mass, and we find no evidence for a
missing population of massive subhalos in the Galaxy. This conclusion is also
supported when we examine the dynamics of the satellites of M31.
