The formation of young dense star clusters through mergers
  Young star clusters like R136 in the Large Magellanic Cloud and NGC 3603,
Westerlund 1, and 2 in the Milky Way are dynamically more evolved than expected
based on their current relaxation times. In particular, the combination of a
high degree of mass segregation, a relatively low central density, and the
large number of massive runaway stars in their vicinity are hard to explain
with the monolithic formation of these clusters. Young star clusters can
achieve such a mature dynamical state if they formed through the mergers of a
number of less massive clusters. The shorter relaxation times of less massive
clusters cause them to dynamically evolve further by the time they merge, and
the merger product preserves the memory of the dynamical evolution of its
constituent clusters. With a series of $N$-body simulations, we study the
dynamical evolution of single massive clusters and those that are assembled
through merging smaller clusters together. We find that the formation of
massive star clusters through the mergers of smaller clusters can reproduce the
currently observed spatial distribution of massive stars, the density, and the
characteristics (number and mass distribution) of the stars ejected as runaways
from young dense clusters. We therefore conclude that these clusters and
possibly other young massive star clusters formed through the mergers of
smaller clusters.
