Measuring the growth of galaxy clusters

We suggest how we can use the mass profile of galaxy clusters beyond their virial radius to measure their mass accretion rate, a key prediction of structure formation models. The mass profile can be estimated by applying the caustic technique to dense redshift surveys of clusters and their outskirts, where dynamical equilibrium does not necessarily hold. An additional probe of the mass growth of clusters is their mass fraction in substructures. We show that the caustic technique, that identifies cluster substructures as a byproduct, returns catalogs of substructures with mass larger than a few 10hM⊙ that are between 60% and 80% complete, depending on the density of the redshift survey.