The treasure behind the haystack: MUSE analysis of five recently discovered globular clusters^⋆

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
e-mail: fgran@oca.eu
² Instituto de Astrofísica, Av. Vicuna Mackenna 4860, Santiago, Chile
³ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 82-0436 Macul, Santiago, Chile
⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Santiago, Chile
⁵ Departamento de Física, Universidad de Santiago de Chile, Av. Victor Jara 3659, Santiago, Chile
⁶ Núcleo Milenio ERIS, Chile
⁷ Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), Universidad de Santiago de Chile, Santiago, Chile
⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
¹⁰ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
¹¹ European Southern Observatory, Karl Schwarzschild-Strabe 2, 85748 Garching bei München, Germany
¹² National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada

Received: 8 September 2023
Accepted: 15 October 2023

Abstract

Context. After the second data release of Gaia, the number of new globular cluster candidates has increased significantly. However, most of them need to be properly characterised, both spectroscopically and photometrically, by means of radial velocities, metallicities, and deeper photometric observations.

Aims. Our goal is to provide an independent confirmation of the cluster nature of Gran 4, a recently discovered globular cluster, with follow-up spectroscopic observations. The derived radial velocity for individual stars, coupled with proper motions, allows us to isolate cluster members from field stars, while the analysis of their spectra allows us to derive metallicities. By including in the analysis the recently confirmed clusters Gran 1, 2, 3, and 5, we aim to completely characterise recently discovered globular clusters.

Methods. Using Gaia DR3 and VVV catalogue data and MUSE at VLT observations, we selected cluster members based on their proper motions, radial velocities and their position in colour-magnitude diagrams. Furthermore, full spectral synthesis was performed on the cluster members, extracting surface parameters and metallicity from MUSE spectra. Finally, a completeness estimation was performed on the total globular cluster population of the Milky Way.

Results. We confirm the nature of Gran 4, a newly discovered globular cluster behind the Galactic bulge, with a mean radial velocity of RV = −265.28 ± 3.92 km s⁻¹ and a mean metallicity of [Fe/H]= − 1.72 ± 0.32 dex. Additionally, independent measurements of the metallicities were derived for Gran 1, 2, 3, and 5. We also revise the observational lower mass limit for a globular cluster to survive in the bulge and disc environment. We estimate that ∼12 − 26 globular clusters have still to be discovered on the other side of the Galaxy (i.e., behind the bulge, bar and disk), up to 20 kpc.