Dependence of Metal Enrichment of Nuclear Star Clusters on Galaxy Stellar Mass

Abstract

Nuclear star clusters (NSCs) are commonly found in the centers of galaxies, but their dominant formation mechanisms remain elusive. We perform a consistent analysis of stellar populations of 97 nearby NSCs, based on spectroscopic data from the Very Large Telescope. The sample covers a galaxy stellar mass range of 10 ^7 –10 ^11 M _⊙ and is more than 3 times larger than any previous study. We identify three galaxy stellar mass regimes with distinct NSC properties. In the low-mass regime of $\mathrm{log}{M}_{{\rm{host}}}$ ≲ 8.5 ( M _host is in units of M _⊙ ), nearly all NSCs have metallicities lower than their circum-NSC host but similar to those of typical red globular clusters (GCs), supporting the GC inspiral–merger scenario of NSC formation. In the high-mass regime of $\mathrm{log}{M}_{{\rm{host}}}$ ≳ 9.5, nearly all NSCs have higher metallicities than their circum-NSC host and red GCs, suggesting significant contributions from in situ star formation. In the intermediate-mass regime, a comparable fraction of NSCs have higher or lower metallicities than their circum-NSC host and red GCs, with no clear dependence on NSC mass, suggesting intermittent in situ star formation. The majority of NSCs with higher metallicities than their host exhibit a negative age–metallicity correlation, providing clear evidence of long-term chemical enrichment. The average metallicity difference between NSC and host peaks broadly around $\mathrm{log}{M}_{{\rm{host}}}\sim 9.8$ and declines toward both higher and lower galaxy masses. We find that the efficiency of dynamical-friction-driven inspiral of GCs observed in present-day galaxies can explain the NSC mass at $\mathrm{log}{M}_{{\rm{host}}}\lesssim 9.5$ but falls short of observed ones at higher galaxy mass, reinforcing our conclusions based on stellar population analysis.

Keywords

• Galaxy nuclei
• Metallicity
• Stellar populations
• Galaxy structure
• Star clusters
• Globular star clusters