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 multi-modal 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 2nd generation, as has
been previously suggested.
