A new low-luminosity globular cluster discovered in the Milky Way with the VVVX survey

Abstract

Context. The VISTA Variables in the Via Láctea Extended Survey (VVVX) allows us to probe previously unexplored regions of the inner Milky Way (MW), especially those that are affected by stellar crowding and strong extinction. Aims. Our long-term goal is to identify new star clusters and investigate them to reveal their true nature. In particular, we are looking for new candidate globular clusters (GCs) located in the Galactic bulge and disk, with the aim of completing the census of the MW GC system. Methods. We searched and characterised new GCs, using a combination of the near-infrared (IR) VVVX survey and Two Micron All Sky survey (2MASS) datasets, and the optical Gaia Early Data Release 3 (EDR3) photometry and its precise proper motions (PMs). Results. We report the discovery of a new Galactic GC, named Garro 02, situated at RA = 18:05:51.1, Dec = −17:42:02 and l = 12.°042, b = +1.°656. Performing a PM-decontamination procedure, we built a final catalogue with all cluster members, on which we performed a photometric analysis. We calculated a reddening of E(J − Ks) = 1.07 ± 0.06 mag and extinction of AKs = 0.79 ± 0.04 mag in the near-IR; while E(BP − RP) = 2.40 ± 0.01 mag and AG = 4.80 ± 0.02 mag in optical passbands. Its heliocentric distance is D = 5.6 ± 0.8 kpc, which places Garro 02 at a Galactocentric distance of RG = 2.9 kpc and Z = 0.006 kpc above the Galactic plane. We also estimated the metallicity and age by comparison with known GCs and by fitting PARSEC isochrones, finding [Fe/H] = −1.30 ± 0.2 dex and age = 12 ± 2 Gyr. We derived the mean cluster PM of (µα*,µδ) = (−6.07 ± 0.62, −6.15 ± 0.75) mas yr−1. We calculated the cluster luminosity in the near-IR of MKs = −7.52 ± 1.23 mag, which is equivalent to MV = −5.44 mag. The core and tidal radii from the radial density profile are rc = 1.25 ± 0.27 arcmin (2.07 pc) and rt = 7.13 ± 3.83 arcmin (11.82 pc), respectively. Conclusions. We confirm Garro 02 as a new genuine Galactic GC, located in the MW bulge. It is a low-luminosity, metal-poor, and old GC, and it is a lucky survivor of the strong dynamical processes that occurred during the MW’s entire life.