2011A&A...527A.124V

Studying the density profiles of galaxy groups offers an important insight into the formation and evolution of the structures in the universe, since galaxy groups bridge the gap between single galaxies and massive clusters. We aim to probe the mass of SL2SJ02140-0535, a galaxy group at z=0.44 from the Strong Lensing Legacy Survey (SL2S), which has uncovered this new population of group-scale strong lenses. We combine strong lensing modeling and dynamical constraints. The strong lensing analysis is based on multi-band HST/ACS observations exhibiting strong lensing features that we have followed-up spectroscopically with VLT/FORS2. To constrain the scale radius of an NFW mass profile that cannot be constrained by strong lensing, we propose a new method by taking advantage of the large-scale dynamical information provided by VLT/FORS2 and KECK/LRIS spectroscopy of group members. In constrast to other authors, we show that the observed lensing features in SL2SJ02140-0535 belong to different background sources: one at z=1.7±0.1 (photometric redshift) produces three images, while the other at z=1.023±0.001 (spectroscopic redshift) has only a single image. Our unimodal NFW mass model reproduces these images very well. It is characterized by a concentration parameter c₂₀₀=6.0±0.6, which is slightly greater than the value expected from ΛCDM simulations for a mass of M₂₀₀≃1x10¹⁴M_☉. The spectroscopic analysis of group members also reveals a unimodal structure that exhibits no evidence of merging. The position angle of the halo is θ=111.6±0.2, which agrees with the direction defined by the luminosity contours. We compare our dynamic mass estimate with an independent weak-lensing based mass estimate finding that both are consistent. Our combined lensing and dynamical analysis of SL2SJ02140-0535 demonstrates the importance of spectroscopic information in reliably identifying the lensing features. Our findings argue that the system is a relaxed, massive galaxy group where mass is traced by light. This work shows a potentially useful method for constraining large-scale properties inaccessible to strong lensing, such as the scale radius of the NFW profile.