Do gravitational lens galaxies have an excess of luminous substructure?
  Strong gravitational lensing can be used to directly measure the mass
function of their satellites, thus testing one of the fundamental predictions
of cold dark matter cosmological models. Given the importance of this test it
is essential to ensure that galaxies acting as strong lenses have dark and
luminous satellites which are representative of the overall galaxy population.
We address this issue by measuring the number and spatial distribution of
luminous satellites in ACS imaging around lens galaxies from the Sloan Lens
Advanced Camera for Surveys (SLACS) lenses, and comparing them with the
satellite population in ACS imaging of non lens galaxies selected from COSMOS,
which has similar depth and resolution to the ACS images of SLACS lenses. In
order to compare the samples of lens and non lens galaxies, which have
intrinsically different stellar mass distributions, we measure, for the first
time, the number of satellites per host as a continuous function of host
stellar mass for both populations. We find that the number of satellites as a
function of host stellar mass, as well as the spatial distribution are
consistent between the samples. Using these results, we predict the number of
satellites we would expect to find around a subset of the Cosmic Lens All Sky
Survey (CLASS) lenses, and find a result consistent with the the number
observed by Jackson et al. 2010. Thus we conclude that within our measurement
uncertainties there is no significant difference in the satellite populations
of lens and non lens galaxies.
