Modelling Star Cluster Formation: Mergers
  Star cluster formation in giant molecular clouds involves the local collapse
of the cloud into small gas-rich subclusters, which can then subsequently
collide and merge to build up the final star cluster(s). In this paper, we
simulate collisions between these subclusters, using coupled smooth particle
hydrodynamics for the gas and N-body dynamics for the stars. We are guided by
previous radiation hydrodynamics simulations of molecular cloud collapse which
provide the global properties of the colliding clusters, such as their stellar
and gas masses, and their initial positions and velocities. The subclusters in
the original simulation were treated as sink particles which immediately merged
into a single entity after the collision. We show that the more detailed
treatment provides a more complex picture. At collisional velocities above ~ 10
km/s, the stellar components of the cluster do not form a monolithic cluster
within 3 Myr, although the gas may do so. At lower velocities, the clusters do
eventually merge but over timescales that may be longer than the time for a
subsequent collision. The structure of the resultant cluster is not well-fit by
any standard density distribution, and the clusters are not in equilibrium but
continue to expand over our simulation time. We conclude that the simple sink
particle treatment of subcluster mergers in large-scale giant molecular cloud
simulations provides an upper limit on the final cluster properties.
