Young Massive Clusters in Merging and Starburst Galaxies

The currently available empirical evidence on the star formation processes in the extreme, high-pressure environments induced by galaxy encounters, mostly based on high-resolution Hubble Space Telescope imaging observations, strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in the starburst events associated with galaxy interactions. The production of young massive star clusters (YSCs) seems to be a hallmark of intense star formation, particularly in interacting and starburst galaxies. Their sizes, luminosities, and mass estimates are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). YSCs are important because of what they can tell us about GC formation and evolution (e.g., initial characteristics and early survival rates). They are also of prime importance as probes of the formation and (chemical) evolution of their host galaxies, and of the initial mass function in the extreme environments required for cluster formation. Recent evidence lends support to the scenario that Milky Way-type GCs (although more metal rich), which were once thought to be the oldest building blocks of galaxies, are still forming today. 1. Extreme environmental conditions Stars rarely form in isolation. In fact, it is well known that the vast majority of stars in the Galaxy, and also in nearby galaxies, are found in groups ranging from small associations, containing some 100 M⊙, to compact, old “globular” and young massive clusters. The nearest examples of these latter objects include the Galactic star-forming regions NGC 3603 and Westerlund 1, and the giant starburst region 30 Doradus with its central star cluster R136 in the Large Magellanic Cloud. Although the older Galactic open clusters (with ages of several Gyr) are undoubtedly gravitationally bound objects, their lower masses compared to the globular cluster population, and more diffuse structures make them much more vulnerable to disk (and bulge) shocking when they pass through the Galactic disk (or close to the bulge), thus leading to enhanced cluster evaporation. These objects are therefore unlikely globular cluster progenitors. It appears that the conditions for the formation of compact, massive star clusters – that have the potential to eventually evolve into globular cluster-type objects by the time they reach a similar age – are currently not present in the Galaxy, or at best to a very limited extent (e.g., Westerlund 1; Hanson 2003). The production of luminous, massive yet compact star clusters seems to be a hallmark of the most intense star-forming episodes, or starbursts. The defining