Infrared photometry and calcium triplet spectroscopy of the most metal-poor in situ globular cluster VVV-CL001^★

¹ Departamento de Astronomia, Facultad de Ciencias, Universidad de La Serena. Av. Raul Bitran 1305, La Serena, Chile
² Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
³ Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
⁴ Instituto de Astronomía Teórica y Experimental (CONICET-UNC), Laprida 854, X5000BGR, Córdoba, Argentina
⁵ Departmento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁶ Vicerrectoría de Investigación y Postgrado, Universidad de La Serena, 1700000, Chile
⁷ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía - Instituto de Astrofísica, Autopista Concepción-Talcahuano 7100, Talcahuano, Chile
⁸ Gemini Observatory NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, Hawaii, 96720, USA
⁹ Rutgers the State University of New Jersey, 136 Frelinghuysen Ave., Piscataway, NJ 08854, USA
¹⁰ Preuniversitario Gauss, Avda. Gabriela Mistral 2868, La Serena, Chile
¹¹ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹³ Instituto de Astrofísica, Departamento de Física y Astronomía, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha, 700, Las Condes, Santiago, Chile

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 July 2025
Accepted: 11 November 2025

Abstract

Context. The characterization of globular clusters (GCs) in the Galactic bulge is a challenging task due to high extinction and severe stellar crowding. VVV-CL001 is a poorly studied GC located in the inner bulge, known for its extremely old age, extreme velocity, and low metallicity. Given its unique properties, a detailed study of this cluster can provide valuable insights into the early chemical and dynamical evolution of the Milky Way (MW).

Aims. The aim of this study was to derive the fundamental parameters of VVV-CL001 including metallicity, heliocentric radial velocity (RV), proper motions (PMs), structural properties, orbit, and age, in order to improve our understanding of its origin and role in the early evolution of the MW.

Methods. We combined spectroscopic, astrometric, and photometric data to characterize VVV-CL001. Metallicity and RV were determined from medium-resolution spectra obtained with FORS2 at the Very Large Telescope. PMs were derived using Gaia DR3 data. Near-infrared photometry from the FourStar instrument on Magellan was used to refine the cluster’s position, construct a radial density profile, and estimate its age, distance, and reddening.

Results. Our results confirm that VVV-CL001 is an old 12.1_−1.2^+1.0 Gyr), metal-poor ([Fe/H] = −2.25 ± 0.05 dex) globular cluster located at a heliocentric position of d_⊙ = 7.1_−1.1^+1.3, with a reddening of E(J − K_s) = 1.40_−0.02^+0.01. Its mean PMs are μ_α^* = −3.68 ± 0.09 mas yr⁻¹ and µ_δ = −1.76 ± 0.10 mas yr⁻¹, and it exhibits a RV of −334 ± 4 km s⁻¹. The cluster follows a retrograde-prograde eccentric (e = 0.76_−0.14^+0.10) orbit, confined within the Galactic plane (|Z|_max = 1.0_−0.32^+0.45 kpc) and inside the bar’s radius of influence (R < 5 kpc), with a pericenter of r_peri = 0.6_−0.2^+0.3 kpc and an apocenter of r_apo = 4.5_−1.2^+2.5 kpc.

Conclusions. These orbital properties, combined with its ancient age and low metallicity, strongly support an in situ origin for VVV-CL001 and likely membership of the disk GC system that was captured by the potential of the bar during its formation. Thus, VVV-CL001 emerges as a fossil remnant of the earliest phases of Galactic assembly and a valuable tracer of the population that contributed to the formation of the inner thick disk and bulge, which are likely part of the main progenitor of the MW. Our study highlights the relevance of detailed chemo-dynamical analyses in unveiling the origin of GCs in the inner Galaxy.