Measuring the total and baryonic mass profiles of the very massive CASSOWARY 31 strong lens. A fossil system at z ~ 0.7?

We investigate the total and baryonic mass distributions in deflector number 31 of the Cambridge And Sloan Survey Of Wide ARcs in the skY (CASSOWARY). We confirm spectroscopically a four-image lensing system at redshift 1.4870 with VLT/X-shooter observations. The lensed images are distributed around a bright early-type galaxy at redshift 0.683, surrounded by several smaller galaxies at similar photometric redshifts. We use available optical and X-ray data to constrain the deflector total, stellar, and hot gas mass through, respectively, strong lensing, stellar population analysis, and plasma modelling. We derive a total mass projected within the Einstein radius R_Ein = 70 kpc of (40 +/- 1) x 10^12 M_Sun, and a central logarithmic slope of -1.7 +/- 0.2 for the total mass density. Despite a very high stellar mass and velocity dispersion of the central galaxy of (3 +/- 1) x 10^12 M_Sun and (450 +/- 80) km/s, respectively, the cumulative stellar-to-total mass profile of the deflector implies a remarkably low stellar mass fraction of 20% (3%-6%) in projection within the central galaxy effective radius R_e = 25 kpc (R = 100 kpc). We also find that the CSWA 31 deflector has properties suggesting it to be among the most distant and massive fossil systems studied so far. The unusually strong central dark matter dominance and the possible fossil nature of this system renders it an interesting target for detailed tests of cosmological models and structure formation scenarios.