Satellites in the field and lens galaxies: SDSS/COSMOS vs. SLACS/CLASS
  The incidence of sub-galactic level substructures is an important quantity,
as it is a generic prediction of high-resolution Cold Dark Matter (CDM) models
which is susceptible to observational test. Confrontation of theory with
observations is currently in an uncertain state. In particular, gravitational
lens systems appear to show evidence for flux ratio anomalies, which are
expected from CDM substructures although not necessarily in the same range of
radius as observed. However, the current small samples of lenses suggest that
the lens galaxies in these systems are unusually often accompanied by luminous
galaxies. Here we investigate a large sample of unlensed elliptical galaxies
from the COSMOS survey, and determine the fraction of objects with satellites,
in excess of background counts, as a function of satellite brightness and
separation from the primary object. We find that the incidence of luminous
satellites within 20 kpc is typically a few tenths of one percent for
satellites of a few tenths of the primary flux, comparable to what is observed
for the wider but shallower SDSS survey. Although the environments of lenses in
the SLACS survey are compatible with this observation, the CLASS radio survey
lenses are significantly in excess of this.
