Spatially Resolving the Star Formation Histories of Three Nearby Nuclear Star Clusters

TL;DR

This study uses spatially resolved spectroscopy to analyze the stellar population gradients in three nearby nuclear star clusters, revealing insights into their formation histories and the processes shaping their structure.

Contribution

It provides the first detailed radial age and metallicity profiles of NSCs, linking stellar populations to formation mechanisms like in situ formation and globular cluster inspiral.

Findings

NGC 5102 NSC shows significant age and metallicity gradients.

Evidence suggests in situ formation for central young populations.

Data indicates a connection between NSC morphology and stellar populations.

Abstract

The formation of nuclear star clusters (NSCs) remains an open question. In this work, we use spatially resolved HST/STIS spectroscopic observations of three nearby NSCs (hosted by NGC 5102, NGC 5206, and NGC 205) to constrain their formation histories by exploring radial variations of the stellar populations within each cluster. Utilizing full-spectrum fitting, we find substantial age and metallicity gradients within the central 0."9 (16~pc) of the NSC in NGC 5102 where populations near the center are young/metal-rich (age ~400~Myr and [M/H] ~-0.4) and become older/metal-poor at larger radii (mean age ~1~Gyr and mean [M/H] ~-1.6 in the radial range [0."3, 0."9]). This behavior suggests that the young/metal-rich population at the center was formed from a period of in situ formation, while the older/metal-poor populations were likely formed by inspiraled globular clusters. The two broad populations observed in the NGC 5102 NSC (young/metal-rich and old/metal-poor) appear to be linked to the transition between the two morphological components of the NSC derived from the surface-brightness profile in Nguyen et al. (2018)(arXiv:1711.04314). The radial ranges explored in NGC 5206 and NGC 205 were much smaller due to poor data quality; in NGC 5206 we find a similar metallicity gradient to NGC 5102 (but with much lower significance), while the data for NGC 205 is too poor to reach any conclusions. Overall, this data highlights the links between the morphological and stellar population complexity of NSCs and their formation mechanisms.