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