Dark matter-rich early-type galaxies in the CASSOWARY 5 strong lensing
  system
  We study the strong gravitational lensing system number 5 identified by the
CASSOWARY survey. In this system, a source at redshift 1.069 is lensed into
four detected images by two early-type galaxies at redshift 0.388. The observed
positions of the multiple images are well reproduced by a model in which the
total mass distribution of the deflector is described in terms of two singular
isothermal sphere profiles. By modelling the lens galaxy spectral energy
distributions, we measure the lens luminous masses and stellar mass-to-light
ratios. These values are used to disentangle the luminous and dark matter
components in the vicinity of the multiple images. We estimate that the dark
over total mass ratio projected within a cylinder centred on the primary lens
and with a radius of 12.6 kpc is 0.8 +/- 0.1. We contrast these measurements
with the typical values found at similar distances (in units of the effective
radius) in isolated lens galaxies and show that the amount of dark matter
present in these lens galaxies is almost a factor four larger than in field
lens galaxies with comparable luminous masses. Data and models are therefore
consistent with interpreting the lens of this system as a galaxy group. We
infer that the overdense environment and dark matter concentration in these
galaxies must have affected the assembly of the lens luminous mass components.
We conclude that further multi-diagnostics analyses on the internal properties
of galaxy groups have the potential of providing us a unique insight into the
complex baryonic and dark-matter physics interplay that rules the formation of
cosmological structures.
