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 _ { \mathrm { Ein } } = 70 kpc of ( 40 \pm 1 ) \times 10 ^ { 12 } M _ { \odot } , and a central logarithmic slope of -1.7 \pm 0.2 for the total mass density .
Despite a very high stellar mass and velocity dispersion of the central galaxy of ( 3 \pm 1 ) \times 10 ^ { 12 } M _ { \odot } and ( 450 \pm 80 ) km s ^ { -1 } , 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 .