Age and metallicity of the Milky Way’s nuclear star cluster studied at 3 pc from Sagittarius A*

Context. The Milky Way’s nuclear star cluster (NSC) offers a unique laboratory for studying the formation and evolution of dense stellar systems around a supermassive black hole. Previous work suggests that most stars in the NSC are old; however, the detailed age and metallicity distributions remain uncertain.
Aims. We aim to constrain the star formation history (SFH) and metallicity of a poorly explored region of the NSC located about 3 pc from Sagittarius A*.
Methods. We analysed NACO/VLT observations in the intermediate-band filter centred at 2.24 μm, complemented by H-band imaging, and constructed colour–magnitude diagrams and completeness-corrected K-band luminosity functions. We clearly identified the red clump and red giant branch bumps. The SFH was derived by fitting cumulative luminosity functions with theoretical models from MIST, PARSEC, and BaSTI, which spans a wide range of ages and metallicities, and by employing Monte Carlo sampling to estimate uncertainties. We also constrained the metallicity of the stellar population, further refined by spectroscopic data from the literature.
Results. The NSC stellar population is predominantly old and metal-rich, with 75.6 ± 9.5% of the stellar mass formed ≳10 Gyr ago and a median metallicity of [M/H] ~ +0.35. Significant contributions come from an intermediate-age population around 2-3 Gyr (20.8 ± 8.7%), while minor components appear at ~400 Myr (0.9 ± 0.8%) and 20 Myr (3.6 ± 1.4%), the latter representing a small but non-negligible young population. Systematic uncertainties related to stellar evolution models, binning, photometric range, unresolved binaries, and filter selection are considered.
Conclusions. Our findings indicate that the NSC formed predominantly in an early episode, with a substantial contribution from a star formation episode 2–3 Gyr ago and minor younger components. The metallicity estimates are consistent with spectroscopic measurements, and the results agree with the stellar population properties of the inner NSC and the inner nuclear stellar disc, providing useful constraints on the transition between these two structures.