Understanding the large inferred Einstein radii of observed low-mass
  galaxy clusters
  We assess a claim that observed galaxy clusters with mass ~10^14 Msun are
more centrally concentrated than predicted in LCDM. We generate mock strong
gravitational lensing observations, taking the lenses from a cosmological
hydrodynamical simulation, and analyse them in the same way as the real
Universe. The observed and simulated lensing arcs are consistent with one
another, with three main effects responsible for the previously claimed
inconsistency. First, galaxy clusters containing baryonic matter have higher
central densities than their counterparts simulated with only dark matter.
Second, a sample of clusters selected because of the presence of pronounced
gravitational lensing arcs preferentially finds centrally concentrated clusters
with large Einstein radii. Third, lensed arcs are usually straighter than
critical curves, and the chosen image analysis method (fitting circles through
the arcs) overestimates the Einstein radii. After accounting for these three
effects, LCDM predicts that galaxy clusters should produce giant lensing arcs
that match those in the observed Universe.
