Hidden in the Haystack: Low-luminosity globular clusters towards the Milky Way bulge

• Published in: arXiv.org 2021-09
• Main Authors: Gran, F, Zoccali, M, Saviane, I, Valenti, E, Rojas-Arriagada, A, R Contreras Ramos, Hartke, J, Carballo-Bello, J A, Navarrete, C, Rejkuba, M, J Olivares Carvajal
• Format: Article
• Language: English
• Subjects: Enceladus; Galactic bulge; Galactic clusters; Globular clusters; High energy astronomy; Luminosity; Orbits; Photometry; Red giant stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2108.11922

Recent wide-area surveys have enabled us to study the Milky Way with unprecedented detail. Its inner regions, hidden behind dust and gas, have been partially unveiled with the arrival of near-IR photometric and spectroscopic datasets. Among recent discoveries, there is a population of low-mass globular clusters, known to be missing, especially towards the Galactic bulge. In this work, five new low-luminosity globular clusters located towards the bulge area are presented. They were discovered by searching for groups in the multi-dimensional space of coordinates, colours, and proper motions from the Gaia EDR3 catalogue and later confirmed with deeper VVV survey near-IR photometry. The clusters show well-defined red-giant branches and, in some cases, horizontal branches with their members forming a dynamically coherent structure in proper motion space. Four of them were confirmed by spectroscopic follow-up with the MUSE instrument on the ESO VLT. Photometric parameters were derived, and when available, metallicities, radial velocities and orbits were determined. The new clusters Gran 1 and 5 are bulge globular clusters, while Gran 2, 3, and 4 present halo-like properties. Preliminary orbits indicate that Gran 1 might be related to the Main Progenitor, or the so-called ''low-energy'' group, while Gran 2, 3 and 5 appear to follow the Gaia-Enceladus-Sausage. This study demonstrates that the Gaia proper motions, combined with the spectroscopic follow-up and colour-magnitude diagrams, are required to confirm the nature of cluster candidates towards the inner Galaxy. High stellar crowding and differential extinction may hide other low-luminosity clusters.