Large Einstein Radii: A Problem for LambdaCDM

Understand

The Einstein radius of a cluster provides a relatively model-independent measure of the mass density of a cluster within a projected radius of ~ 150 kpc, large enough to be relatively unaffected by gas physics.

• We show that the observed Einstein radii of four well-studied massive clusters, for which reliable virial masses are measured, lie well beyond the predicted distribution of Einstein radii in the standard LambdaCDM model.
• Based on large samples of numerically simulated cluster-sized objects with virial masses ~ 1e15 solar, the predicted Einstein radii are only 15-25'', a factor of two below the observed Einstein radii of these four clusters.
• This is because the predicted mass profile is too shallow to exceed the critical surface density for lensing at a sizable projected radius.