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

We report the first star formation history study of the Milky Ways nuclear star cluster (NSC), which includes observational constraints from a large sample of stellar metallicity measurements. These metallicity measurements were obtained from recent surveys from Gemini and the Very Large Telescope 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% +/- 3% of the mass, is metal-rich (([M H]) over bar similar to 0.45), and has an age of 5(-2)(+3) Gyr, which is similar to 3 Gyr younger than earlier studies with fixed (solar) 3-metallicity; this younger age challenges coevolutionary models in which the NSC and supermassive black holes formed simultaneously at early times. The minor population component has low metallicity (([M/H]) over bar similar to -1.1) and contains similar to 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 similar to 18% uncertainty): 1.5 x 10(5) neutron stars, 2.5 x 10(5) stellar-mass black holes (BHs), and 2.2 x 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)).

Automated summary

We present the first study on the star formation history of the Milky Way's nuclear star cluster (NSC), incorporating observational constraints from a large sample of stellar metallicity measurements. The inclusion of metallicity measurements improves the overall fit quality and suggests a two-component model for the NSC. The dominant component is metal-rich and younger than previously thought, challenging coevolutionary models. The minor population component has low metallicity and an uncertain age. The study also provides predictions for the NSC stellar remnant population, including neutron stars and stellar-mass black holes.