Abell 2744: Too much substructure for Lambda CDM?
  The massive substructures found in Abell 2744 by Jauzac et al. (2016) present
a challenge to the cold dark matter paradigm due to their number and proximity
to the cluster centre. We use one of the biggest N-body simulations, the
Millennium XXL, to investigate the substructure in a large sample of massive
dark matter haloes. A range of effects which influence the comparison with the
observations is considered, extending the preliminary evaluation carried out by
Jauzac et al. (2016). There are many tens of haloes in the simulation with a
total mass comparable with or larger than that of Abell 2744. However, we find
no haloes with a number and distribution of massive substructures (> 5 10^13
Msun) that is close to that inferred from the observations of Abell 2744. The
application of extreme value statistics suggests that we would need a
simulation of at least ten times the volume of the Millennium XXL to find a
single dark matter halo with a similar internal structure to Abell 2744.
Explaining the distribution of massive substructures in clusters is a new
hurdle for hierarchical models to negotiate, which is not weakened by appeals
to baryonic physics or uncertainty over the nature of the dark matter particle.
