Starburst clusters and their environment
About a handful of young, massive star clusters in the Milky Way contain the 10,000 solar masses to qualify as starburst clusters. Although the known sample is far from complete, about half of these massive clusters are found near the center of the Galaxy, while the remainder is located in spiral arms, where star formation is abundant. One of the three central young clusters has formed 6 Myr ago in the immediate vicinity of the supermassive black hole, the nuclear cluster (e.g., Schoedel et al. 2007, and references therein). The Arches and Quintuplet clusters are located at projected distances of ~30 pc from the Galactic centre. The finding of these few million year young objects so close to the center of the Galaxy is puzzling. Tidal shear forces from the strong gravitational field caused by the dense stellar population in the central region of the Galaxy, as well as the strong magnetic and UV radiation fields, should prohibit stellar fragmentation in the externally heated molecular clouds. The high productivity of starburst clusters raises the question whether these clusters form in a similar mode as their spiral arm cousins, or whether different physical processes and formation mechanisms have to be invoked to explain their existence.
For the Galactic centre Arches cluster in particular, the large orbital motion is difficult to explain from the properties of known dense molecular clouds in the GC. How did the cluster inherit an orbital velocity of 230 km/s? Can cloud collisions explain the puzzling cluster properties, and are they required to form GC clusters? Do they exhibit dynamical properties substantially different from spiral arm clusters?
As a consequence of the external heating sources, fragmentation is more difficult than in spiral arm clouds, and GC clusters are predicted to form an excess of high-mass stars. The fast dynamical disruption in the tidal field, however, renders a prestine measurement of the stellar mass distribution originating in these clusters very challenging. With the recent achievement of proper motion membership selections, the derivation of the initial stellar mass distribution is now within reach and will enable an advancement regarding the differences and similarities in the emerging population of spiral arm and central clusters. Understanding these differences will be crucial not only for our knowledge of Milky Way star and cluster formation, but will be especially
valueable for our understanding of unresolved, extragalactic cluster populations.
Arches
Quintuplet
Young Nuclear Cluster
NGC 3603
Westerlund 1 & 2
Approximate locations of Milky Way starburst clusters.
Image courtesy Wikipedia.
Sun