Dark matter distribution in galaxy groups from combined strong lensing
  and dynamics analysis
  Using a combined analysis of strong lensing and galaxy dynamics, we
characterize the mass distributions and M/L ratios of galaxy groups, which form
an important transition regime in Lambda-CDM cosmology. By mapping the
underlying mass distribution, we test whether groups are dark matter dominated
as hypothesized by the standard cosmogony, or isothermal as observed in baryon
rich field galaxies. We present our lensing + galaxy dynamics formalism built
around the dark matter dominant NFW and Hernquist distributions, compared
against the Isothermal Sphere observed in galaxy scale objects. We show that
mass measurement in the core of the group (r ~ 0.2 r_{vir}), determined jointly
from a lens model and from differential velocity dispersion estimates, may
effectively distinguish between these density distributions. We apply our
method to MOS observations of two groups, SL2SJ1430+5546 and SL2SJ1431+5533,
drawn from our CFHTLS lens catalog. With the measured lensing and dynamical
masses, combined with a maximum likelihood estimator built around our model, we
estimate the concentration index characterizing each density distribution and
the corresponding virial mass of each group. Our results indicate that both
groups are dark matter dominant, and reject the Isothermal distribution at >>3
sigma level. For both groups, the estimated i-band M/L ratios of ~260
Msun/Lsun, are similar to other published values for groups. The Gaussian
distributions of the velocities of their member galaxies support a high degree
of virialization. The differences in their virial masses, 2.8 and 1.6 x 10^14
Msun, and velocity dispersions, 720 and 560 km/s respectively, may indicate
however that each group is at a different stage of transition to a cluster. We
aim to populate this important transition regime with additional results from
ongoing observations of the remaining lensing groups in our catalog.
