The population of Milky Way satellites in the LambdaCDM cosmology
  We present a model for the satellites of the Milky Way in which galaxy
formation is followed using semi-analytic techniques applied to the six
high-resolution N-body simulations of galactic halos of the Aquarius project.
The model, calculated using the Galform code, incorporates improved treatments
of the relevant physics in the LambdaCDM cosmogony, particularly a
self-consistent calculation of reionization by UV photons emitted by the
forming galaxy population, including the progenitors of the central galaxy.
Along the merger tree of each halo, the model calculates gas cooling (by
Compton scattering off cosmic microwave background photons, molecular hydrogen
and atomic processes), gas heating (from hydrogen photoionization and supernova
energy), star formation and evolution. The evolution of the intergalactic
medium is followed simultaneously with that of the galaxies. Star formation in
the more massive progenitor subhalos is suppressed primarily by supernova
feedback, while for smaller subhalos it is suppressed primarily by
photoionization due to external and internal sources. The model is constrained
to match a wide range of properties of the present day galaxy population as a
whole, but at high redshift it requires an escape fraction of UV photons near
unity in order completely to reionize the universe by redshift z ~ 8. In the
most successful model the local sources photoionize the pre-galactic region
completely by z ~ 10. In addition to the luminosity function of Milky Way
satellites, the model matches their observed luminosity-metallicity relation,
their radial distribution and the inferred values of the mass within 300 pc,
which in the models increase slowly but significantly with luminosity. There is
a large variation in satellite properties from halo to halo, with the
luminosity function, for example, varying by a factor of ~ 2 among the six
simulations.
