2010ApJ...714.1355T

Using a combined analysis of strong lensing and galaxy dynamics, we characterize the mass distributions and the mass-to-light (M/L) ratios of galaxy groups, virialized structures in the mass range of fewx10¹⁴ M _☉, which form an important transition regime in the hierarchical assembly of mass in ΛCDM cosmology. Our goals are to not only map the mass distributions, but to also test whether the underlying density distribution at this mass scale is dark matter dominated, Navarro-Frenk-White (NFW) like as hypothesized by the standard cosmogony, or isothermal as observed in baryon-rich massive field galaxies. We present details of our lensing + galaxy dynamics formalism built around three representative density profiles, the dark matter dominant NFW and Hernquist distributions, compared with the softened isothermal sphere which matches baryon-rich galaxy scale objects. By testing the effects on the characteristics of these distributions due to variations in their parameters, we show that mass measurements in the core of the group (r/r _vir∼ 0.2), determined jointly from a lens model and from differential velocity dispersion estimates, may effectively distinguish between these density distributions. We apply our method to multi-object spectroscopy observations of two groups, SL2SJ143000+554648 and SL2SJ143139+553323, drawn from our catalog of galaxy group scale lenses discovered in CFHTLS-Wide imaging. With the lensing and dynamical mass estimates from our observations along 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 likelihood estimation indicates that both groups are dark matter dominant and rejects the isothermal distribution at {Gt}3σ level. For both groups, the estimated i-band M/L ratios of ∼260 M_☉ L_☉^–1 are similar to other published values for groups. The Gaussian distribution of the velocities of their member galaxies supports a high degree of virialization. The differences in their virial masses, 2.8 and 1.6x10¹⁴ M_☉, and velocity dispersions, 720 and 560 km/s, 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.