Chemical analysis of the Milky Way’s nuclear star cluster

Context. The Milky Way nuclear star cluster (MWNSC) is located in the Galactic centre, together with the Milky Way nuclear stellar disc (MWNSD), and they dominate the gravitational potential within the inner 300 pc. However, the formation and evolution of the two systems and their possible connections are still under debate. Aims. We reanalysed the low-resolution KMOS spectra in the MWNSC with the aim of improving the stellar parameters (Teff, log g, and [M/H]) for the MWNSC. Methods. We used an improved line list, especially dedicated for cool M giants, that allowed us to improve the stellar parameters and to obtain in addition global α-elements. A comparison with high-resolution IR spectra (from IGRINS) gives very satisfactory results and constrains the uncertainties to Teff ≃ 150 K, log g ≃ 0.4 dex, and [M/H] ≃ 0.2 dex. Our α-elements agree within 0.1 dex compared to the IGRINS spectra. Results. We obtained a high-quality sample of 1140 M giant stars where we see an important contribution of a metal-poor population (∼20%) centred at [M/H] ≃−0.7 dex, while the most dominant part comes from the metal-rich population with [M/H] ≃ 0.26 dex. We constructed a metallicity map and find a metallicity gradient of ∼−0.1 ± 0.02 dex/pc favouring the inside-out formation scenario for the MWNSC.