Dark matter fraction of low-mass cluster members probed by galaxy-scale strong lensing

We present a strong lensing system, composed of four multiple images of a source at z = 2.387, created by two lens galaxies, G1 and G2, belonging to the galaxy cluster MACS J1115.9+0129 at z = 0.353. We use observations taken as part of the Cluster Lensing and Supernova survey with Hubble, and its spectroscopic follow-up programme at the Very Large Telescope, to estimate the total mass distributions of the two galaxies and the cluster through strong gravitational lensing models. We find that the total projected mass values within the half-light radii, Re, of the two lens galaxies are MT,G1(<Re, G1) = (3.6 ± 0.4) × 1010 M⊙ and MT,G2(<Re, G2) = (4.2 ± 1.6) × 1010 M⊙. The effective velocity dispersion values of G1 and G2 are (122 ± 7) km s-1 and (137 ± 27) km s-1, respectively. We remark that these values are relatively low when compared to those of ≈200-300 km s-1, typical of lens galaxies found in the field by previous surveys. By fitting the spectral energy distributions of G1 and G2, we measure projected luminous over total mass fractions within Re of 0.11 ± 0.03, for G1, and 0.73 ± 0.32, for G2. The fact that the less massive galaxy, G1, is dark matter-dominated in its inner regions raises the question of whether the dark matter fraction in the core of early-type galaxies depends on their mass. Further investigating strong lensing systems will help us understand the influence that dark matter has on the structure and evolution of the inner regions of galaxies.