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

Zhuo ChenDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USATuan DoDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USAA. M. GhezDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USAMatthew HosekDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USAA. FeldmeierMax Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, GermanyDevin S. ChuDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USARory O. BentleyDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USAJessica R. LuDepartment of Astronomy, University of California, Berkeley, CA 94720-3411, USAM. MorrisDepartment of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA

2023en

ABI

Annotatsiya

Abstract 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">M</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> <mml:mrow> <mml:mo stretchy="true">¯</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∼</mml:mo> <mml:mn>0.45</mml:mn> </mml:math> ), and has an age of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> Gyr, which is ∼3 Gyr younger than earlier studies with fixed (solar) 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">M</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> <mml:mrow> <mml:mo stretchy="true">¯</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∼</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.1</mml:mn> </mml:math> ) and contains ∼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 ∼18% uncertainty): 1.5 × 10 5 neutron stars, 2.5 × 10 5 stellar-mass black holes (BHs), and 2.2 × 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 ).

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DOI: 10.3847/1538-4357/aca8ad

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