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 $( l,b = 5.26^̧irc,0.77^̧irc )$ is perhaps the most metal-poor (̊m Fe/H ∼-2.2 dex) globular cluster in the Galactic bulge and a promising candidate to be the oldest one in the entire Milky Way. Due to its recent discovery and being behind a thick veil of interstellar clouds, 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, study its role in the formation of the Milky Way, and determine whether it is the oldest Galactic globular cluster. We used and observations to construct the deepest color-magnitude diagram (CMD) of VVV CL001 to date based on FourStar/Baade near-infrared (NIR) photometry combined with Hubble Space Telescope (HST) NIR 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 five-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 turnoff yielded the age from different isochrone sets. Finally, we refined the cluster's orbital parameters. We obtained a distance of d=7.34 kpc, the most precise distance reported in the literature so far. We derived an age of 13.4 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.