VVV CL001: A strong candidate for the most ancient star cluster in the Galaxy
Abstract
Finding the oldest globular cluster of the Galaxy is important to constrain a lower limit on the age of the Galaxy and the Universe. VVV CL001 ([l,b]= [5.26,0.77]) is perhaps the most metal-poor ([Fe/H]~-2.2 dex) globular cluster in the Galactic bulge and a promising candidate to be the oldest globular cluster in the entire Milky Way. Due to its recent discovery and high extinction, its physical parameters, especially the distance and age, remain highly uncertain, yet they are essential to define it as an ancient native stellar system of the Milky Way. We aim to refine the distance and age of VVV CL001 using deep and high-quality photometry, to study its role in the formation of the Milky Way, and to determine whether it is the oldest Galactic globular cluster. We used J and KS observations to construct the deepest CMD of VVV CL001 to date, based on FourStar/Baade PSF photometry combined with HST near-IR wide-filter PSF photometry. We selected cluster members through proper motion calculations and the HDBSCAN algorithm, and assigned membership probabilities via Markov-Chain Monte Carlo iterations in a 5-dimensional parameter space. Using r band data from DECam/Blanco, we optically identified horizontal branch members and applied an isochrone-fitting technique to determine the cluster distance, while members around the main-sequence turn-off yielded the age from different isochrone sets. Finally, we refined the cluster orbital parameters. We obtained a distance of d = 7.34 +- 0.46 kpc, the most precise to date. We derived an age of 13.4 +- 0.4 Gyr, indicating that VVV CL001 is one of the oldest globular clusters in the Galaxy. The orbit confirms that the cluster belongs to the Galactic bulge, which, together with its age and metallicity, could link it to the early proto-Milky Way assembly, extending the metal-poor end of the known in situ globular clusters.
Related
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BibTeX
@misc{carvajal2026cl001,
title = {{VVV CL001: A strong candidate for the most ancient star cluster in the Galaxy}},
author = {J. Olivares Carvajal and Á. Valenzuela Navarro and C. Salvo-Guajardo and C. Ordenes-Huanca and B. Acosta-Tripailao and C. Quezada and F. Gran and M. Zoccali and M. Dall'Ora and M. De Leo and S. Villanova and D. Geisler},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.27969},
eprint = {2610.27969},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.27969},
}
RIS
TY - UNPB
TI - VVV CL001: A strong candidate for the most ancient star cluster in the Galaxy
AU - J. Olivares Carvajal
AU - Á. Valenzuela Navarro
AU - C. Salvo-Guajardo
AU - C. Ordenes-Huanca
AU - B. Acosta-Tripailao
AU - C. Quezada
AU - F. Gran
AU - M. Zoccali
AU - M. Dall'Ora
AU - M. De Leo
AU - S. Villanova
AU - D. Geisler
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2610.27969
UR - https://arxiv.org/abs/2610.27969
AB - Finding the oldest globular cluster of the Galaxy is important to constrain a lower limit on the age of the Galaxy and the Universe. VVV CL001 ([l,b]= [5.26,0.77]) is perhaps the most metal-poor ([Fe/H]~-2.2 dex) globular cluster in the Galactic bulge and a promising candidate to be the oldest globular cluster in the entire Milky Way. Due to its recent discovery and high extinction, its physical parameters, especially the distance and age, remain highly uncertain, yet they are essential to define it as an ancient native stellar system of the Milky Way. We aim to refine the distance and age of VVV CL001 using deep and high-quality photometry, to study its role in the formation of the Milky Way, and to determine whether it is the oldest Galactic globular cluster. We used J and KS observations to construct the deepest CMD of VVV CL001 to date, based on FourStar/Baade PSF photometry combined with HST near-IR wide-filter PSF photometry. We selected cluster members through proper motion calculations and the HDBSCAN algorithm, and assigned membership probabilities via Markov-Chain Monte Carlo iterations in a 5-dimensional parameter space. Using r band data from DECam/Blanco, we optically identified horizontal branch members and applied an isochrone-fitting technique to determine the cluster distance, while members around the main-sequence turn-off yielded the age from different isochrone sets. Finally, we refined the cluster orbital parameters. We obtained a distance of d = 7.34 +- 0.46 kpc, the most precise to date. We derived an age of 13.4 +- 0.4 Gyr, indicating that VVV CL001 is one of the oldest globular clusters in the Galaxy. The orbit confirms that the cluster belongs to the Galactic bulge, which, together with its age and metallicity, could link it to the early proto-Milky Way assembly, extending the metal-poor end of the known in situ globular clusters.
ER -