Star Cluster Ecology: Revisiting the Origin of Iron and Age Complex Clusters

TL;DR

This paper explores the origins of complex stellar populations in certain high-mass clusters, challenging the idea they are remnants of galactic bulge formation and proposing they are likely formed through gas accretion and second-generation star formation.

Contribution

It critically evaluates the hypothesis that these clusters are bulge remnants and supports the alternative scenario of gas accretion and secondary star formation in GCs.

Findings

Dynamical friction likely invalidates the bulge remnant hypothesis.

The initial mass estimation method used for the clusters was flawed.

These clusters probably formed through gas accretion and second-generation star formation.

Abstract

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.