Connecting substructure in galaxy cluster cores at z=0.2 with cluster assembly histories

We use semianalytic models of structure formation to interpret gravitational lensing measurements of substructure in galaxy cluster cores (R 10% mass growth in the similar to 500 Myr preceding z = 0.2, i.e., since z = 0.25. In summary, observational measurements of fsub can be combined with structure formation models to estimate the age and assembly history of observed clusters. The ability to "age date" approximately clusters in this way has numerous applications to the large samples of clusters that are becoming available.