A MUSE study of the inner bulge globular cluster Terzan 9: a fossil record in the Galaxy

Moderately metal-poor inner bulge globular clusters are relics of a generation of long-lived stars formed in the early Galaxy. Aims. Terzan 9, projected at only 4◦.12 of the Galactic center, shows an orbit remaining confined in the inner 1 kpc of the Galaxy center, and is among the most central globular clusters in the Milky Way. Metallicities in the range -2.0<[Fe/H]<-1.2 were estimated for Terzan 9 in the literature, from Colour-Magnitude Diagrams and CaII triplet (CaT) lines. Therefore derivation of metallicity is a primary goal. Given its compactness, Terzan 9 was observed with the Multi Unit Spectroscopic Explorer (MUSE), at the Very Large Telescope. Extraction of several hundreds of cluster spectra allow to derive metallicities and [Mg/Fe] based on the Mg2 feature. The spectra obtained with MUSE were analysed through full spectrum fitting, with the code Etoile