Triggering the Formation of Massive Clusters
  There are at least 2 distinct mechanisms for the formation of young massive clusters (YMC), all of which require galactic-scale processes. One operates in harrassed fragile galaxies, in the dense cores of low mass galaxies, at the ends of spiral arms, or in galactic tidal shocks where transient and peculiar high pressures make massive clouds at high densities. The result of this process is usually only one or two YMC without the usual morphologies of local star formation, i.e., without hierarchical structure and a continuous power law distribution of cluster masses up to the largest mass. The other operates in the more usual way: continuously for long periods of time in large parts of the interstellar medium where the ambient pressure is already high as a result of the deep potential well from background stars and other gas. This second process makes clusters in a hierarchical fashion with size-of-sample effects, and tends to occur in nuclear rings, merger remnants, and even the ambient ISM of normal galaxies if the star formation rate is high enough to sample out to the YMC range. 
Bruce G. Elmegreen (IBM T.J. Watson Research Center)