The Star Formation History of the Milky Way's Nuclear Star Cluster

TL;DR

This study reconstructs the star formation history of the Milky Way's nuclear star cluster using metallicity data, revealing a younger, metal-rich dominant population and implications for black hole and neutron star populations.

Contribution

It introduces a Bayesian modeling approach incorporating metallicity measurements, revealing a younger, metal-rich dominant population and refining stellar remnant predictions.

Findings

Dominant population is metal-rich (~0.45) and ~3 Gyr younger than previous estimates.

Minor population is low metallicity (~-1.1) with uncertain age.

Predicted neutron star and black hole populations are 2-4 times lower than earlier models.

Abstract

We report the first star formation history study of the Milky Way's nuclear star cluster (NSC) that includes observational constraints from a large sample of stellar metallicity measurements. These metallicity measurements were obtained from recent surveys from Gemini and VLT of 770 late-type stars within the central 1.5 pc. These metallicity measurements, along with photometry and spectroscopically derived temperatures, are forward modeled with a Bayesian inference approach. Including metallicity measurements improves the overall fit quality, as the low-temperature red giants that were previously difficult to constrain are now accounted for, and the best fit favors a two-component model. The dominant component contains 93%$\pm$3% of the mass, is metal-rich ($\overline{[M/H]}\sim$0.45), and has an age of 5$^{+3}_{-2}$ Gyr, which is $\sim$3 Gyr younger than earlier studies with fixed (solar) metallicity; this younger age challenges co-evolutionary models in which the NSC and supermassive black holes formed simultaneously at early times. The minor population component has low metallicity ($\overline{[M/H]}\sim$ -1.1) and contains $\sim$7% of the stellar mass. The age of the minor component is uncertain (0.1 - 5 Gyr old). Using the estimated parameters, we infer the following NSC stellar remnant population (with $\sim$18% uncertainty): 1.5$\times$10$^5$ neutron stars, 2.5$\times$10$^5$ stellar mass black holes (BHs) and 2.2$\times$10$^4$ BH-BH binaries. These predictions result in 2-4 times fewer neutron stars compared to earlier predictions that assume solar metallicity, introducing a possible new path to understand the so-called "missing pulsar problem". Finally, we present updated predictions for the BH-BH merger rates (0.01-3 Gpc$^{-3}$yr$^{-1}$).