The lens SW05 J143454.4+522850: a fossil group at redshift 0.6?
  Fossil groups are considered the end product of natural galaxy group
evolution in which group members sink towards the centre of the gravitational
potential due to dynamical friction, merging into a single, massive, and X-ray
bright elliptical. Since gravitational lensing depends on the mass of a
foreground object, its mass concentration, and distance to the observer, we can
expect lensing effects of such fossil groups to be particularly strong. This
paper explores the exceptional system $\mathrm{J}143454.4+522850$. We combine
gravitational lensing with stellar population-synthesis to separate the total
mass of the lens into stars and dark matter. The enclosed mass profiles are
contrasted with state-of-the-art galaxy formation simulations, to conclude that
SW05 is likely a fossil group with a high stellar to dark matter mass fraction
$0.027\pm0.003$ with respect to expectations from abundance matching
$0.012\pm0.004$, indicative of a more efficient conversion of gas into stars in
fossil groups.
