Star cluster ecology: revisiting the origin of iron and age complex clusters

Typical globular clusters (GCs - young and old) host stellar populations with little or no star-to-star variations in heavy elements (e.g. Ca, Fe) nor in age. Nuclear star clusters (NSCs), on the other hand, host complex stellar populations that show multimodal distributions in Fe and often in age, presumably due to their unique location at the centre of a large galactic potential well. However, recently a new class of clusters have been discovered, exemplified by the clusters Terzan 5 and Liller 1, two high mass, high metallicity clusters in the inner Galactic regions. It has been suggested that these are not true GCs, but rather represent left over fragments of the formation of the Galactic Bulge. Here, we critically assess this scenario and find that the role of dynamical friction likely makes it untenable and that the method used to estimate the initial masses of the clumps was invalid. Instead, it appears more likely that these clusters represent a relatively rare phenomenon of existing GCs accreting gas and forming a second generation, as has been previously suggested.

Automated summary

Traditional globular clusters and nuclear star clusters have distinct characteristics, while newly discovered clusters are believed to be remnants of the formation of the Galactic Bulge. However, it is found that this assumption is unlikely to hold, and it is more likely that these clusters accrete gas to form a second generation.