Super-star clusters versus OB associations
  Super Star Clusters (Mecl > 10^5 Msol) are the largest stellar nurseries in
our local Universe, containing hundreds of thousands to millions of young stars
within a few light years. Many of these systems are found in external galaxies,
especially in pairs of interacting galaxies, and in some dwarf galaxies, but
relatively few in disk galaxies like our own Milky Way. We show that a possible
explanation for this difference is the presence of shear in normal spiral
galaxies which impedes the formation of the very large and dense super star
clusters but prefers the formation of loose OB associations possibly with a
less massive cluster at the center. In contrast, in interacting galaxies and in
dwarf galaxies, regions can collapse without having a large-scale sense of
rotation. This lack of rotational support allows the giant clouds of gas and
stars to concentrate into a single, dense and gravitationally bound system.
