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

Chemical characterization of the Galactic center is essential for understanding its formation and structural evolution. Trends of alpha (α) elements, such as magnesium, silicon, and calcium, 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 α-element trends with metallicity. High-resolution, near-infrared spectra were obtained using the Immersion GRating INfrared 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 α-element abundances in the predominantly metal-rich NSC stars, consistent with trends in the inner bulge. The NSC stars follow the high-[α/Fe] envelope seen in the solar vicinity's metal-rich population, indicating a high star formation rate. The α-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.