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

TL;DR

This study uses MUSE spectroscopy to determine the metallicity, radial velocity, and chemical composition of the inner bulge globular cluster Terzan 9, revealing its status as a moderately metal-poor, ancient stellar relic in the Galaxy.

Contribution

First detailed spectroscopic analysis of Terzan 9 using MUSE, providing precise metallicity and velocity measurements that clarify its nature and relation to other bulge clusters.

Findings

Mean metallicity [Fe/H] = -1.10 ± 0.15

Radial velocity v_hr = 58.1 ± 1.1 km/s

[Mg/Fe] = 0.27 ± 0.03

Abstract

Context. Moderately metal-poor inner bulge globular clusters are relics of a generation of long-lived stars that formed in the early Galaxy. Terzan 9, projected at 4d 12 from the Galactic center, is among the most central globular clusters in the Milky Way, showing an orbit which remains confined to the inner 1 kpc. Aims. Our aim is the derivation of the cluster's metallicity, together with an accurate measurement of the mean radial velocity. In the literature, metallicities in the range between have been estimated for Terzan 9 based on color-magnitude diagrams and CaII triplet (CaT) lines. Aims. Our aim is the derivation of the cluster's metallicity, together with an accurate measurement of the mean radial velocity. In the literature, metallicities in the range between -2.0 and -1.0 have been estimated for Terzan 9 based on color-magnitude diagrams and CaII triplet (CaT) lines. Methods. Given its compactness, Terzan 9 was observed using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope. The extraction of spectra from several hundreds of individual stars allowed us to derive their radial velocities, metallicities, and [Mg/Fe]. The spectra obtained with MUSE were analysed through full spectrum fitting using the ETOILE code. Results. We obtained a mean metallicity of [Fe/H] -1.10 0.15, a heliocentric radial velocity of vhr = 58.1 1.1 km/s , and a magnesium-to-iron [Mg/Fe] = 0.27 0.03. The metallicity-derived character of Terzan 9 sets it among the family of the moderately metal-poor Blue Horizontal Branch clusters HP 1, NGC 6558, and NGC 6522.