CSWA 31 MUSE field spectroscopic redshifts

Our best-fit mass models reproduce the positions of multiple images and provide robust reconstructions for two background galaxies at z=1.4869 and z=2.763. Despite small variations related to the different sets of input constraints, the relative contributions from the BGG and group-scale halo are remarkably consistent in our three reference models, demonstrating the self- consistency between strong lensing analyses based on image position and extended image modeling.

We find that the ultra-massive BGG dominates the projected total mass profiles within 20kpc, while the group-scale halo dominates at larger radii. The total projected mass enclosed within Reff=27.2kpc is 1.10^+0.02^_-0.04_*10^13^M_{sun}_. We find that CSWA 31 is a peculiar fossil group, strongly dark-matter dominated towards the central region, and with a projected total mass profile similar to higher-mass cluster-scale halos.

The total mass-density slope within the effective radius is shallower than isothermal, consistent with previous analyses of early-type galaxies in overdense environments.