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.
