Chemical Abundances in the Nuclear Star Cluster of the Milky Way: alpha-Element Trends and Their Similarities with the Inner Bulge

TL;DR

This study analyzes alpha-element abundances in the Milky Way's Nuclear Star Cluster using high-resolution infrared spectra, revealing similarities with the inner bulge and insights into its formation history.

Contribution

It provides the first detailed chemical abundance analysis of NSC M giants, linking their alpha-element trends to the inner bulge and galaxy evolution.

Findings

NSC stars show enhanced alpha-element abundances similar to the inner bulge.

Alpha-element trends decrease with increasing metallicity.

NSC shares a similar evolutionary history with the inner bulge.

Abstract

A chemical characterization of the Galactic Center is essential for understanding its formation and structural evolution. Trends of alpha-elements, such as Mg, Si, and Ca, serve as powerful diagnostic tools, offering insights into star-formation rates and gas-infall history. However, high extinction has previously hindered such studies. In this study, we present a detailed chemical abundance analysis of M giants in the Milky Way's Nuclear Star Cluster (NSC), focusing on alpha-element trends with metallicity. High-resolution, near-infrared spectra were obtained using the IGRINS spectrograph on the Gemini South telescope for nine M giants. Careful selection of spectral lines, based on a solar-neighborhood control sample of 50 M giants, was implemented to minimize systematic uncertainties. Our findings show enhanced alpha-element abundances in the predominantly metal-rich NSC stars, consistent with trends in the inner bulge. The NSC stars follow the high-[alpha/Fe] envelope seen in the solar vicinity's metal-rich population, indicating a high star-formation rate. The alpha-element trends decrease with increasing metallicity, also at the highest metallicities. Our results suggest the NSC population likely shares a similar evolutionary history with the inner bulge, challenging the idea of a recent dominant star formation burst. This connection between the NSC and the inner-disk sequence suggests that the chemical properties of extragalactic NSCs of Milky Way type galaxies could serve as a proxy for understanding the host galaxies' evolutionary processes.