Hierarchical formation of Westerlund 1: a collapsing cluster with no
  primordial mass segregation?
  We examine the level of substructure and mass segregation in the massive,
young cluster Westerlund 1. We find that it is relatively smooth, with little
or no mass segregation, but with the massive stars in regions of significantly
higher than average surface density. While an expanding or bouncing-back
scenario for the evolution of Westerlund 1 cannot be ruled out, we argue that
the most natural model to explain these observations is one in which Westerlund
1 formed with no primordial mass segregation and at a similar or larger size
than we now observe.
