Spatial and luminosity distributions of galactic satellites
  We investigate the luminosity functions (LFs) and projected number density
profiles of galactic satellites around isolated primaries of different
luminosities. We measure these quantities for model satellites placed into the
Millennium and Millennium II dark matter simulations by the GALFORM
semi-analytic galaxy formation model for different bins of primary galaxy
magnitude and we investigate their dependence on satellite luminosity. We
compare our model predictions to the data of Guo et al. from the Sloan Digital
Sky Survey Data Release 8 (SDSS DR8). First, we use a mock light-cone catalogue
to verify that the method we used to count satellites in the SDSS DR8 is
unbiased. We find that the radial distributions of model satellites are similar
to those around comparable primary galaxies in the SDSS DR8, with only slight
differences at low luminosities and small projected radii. However, when
splitting the satellites by colour, the model and SDSS satellite systems no
longer resemble one another, with many red model satellites, in contrast to the
dominant blue fraction at similar luminosity in SDSS. The few model blue
satellites are also significantly less centrally concentrated in the halo of
their stacked primary than their SDSS counterparts. The implications of this
result for the GALFORM model are discussed.
