A newly discovered VVVX star cluster surfing the Galactic disk

Abstract

We report Garro 04, a previously unidentified stellar cluster toward the Galactic disk, which was discovered as part of the ESO/KMOS VVVX-GalCen Spectroscopic Survey. This object was serendipitously confirmed as a stellar overdensity in the VISTA Variables in the Vía Láctea eXtended (VVVX) survey footprint and subsequently confirmed through medium-resolution K-band spectroscopy obtained with the K-band Multi Object Spectrograph (KMOS) at the ESO Very Large Telescope. The combined photometric and spectroscopic analysis reveals a coherent stellar population with common radial velocities and chemical properties, supporting its classification as a genuine star cluster rather than a statistical fluctuation of the disk field. We derived the fundamental properties of Garro 04, including its structural parameters, distance, and kinematics, and computed its orbital properties, finding that its motion remains confined to the Galactic disk region. Finally, our analysis suggests that Garro 04 is likely an old open cluster or an object straddling the boundary between the canonical globular and open cluster classifications, with an estimated age of 5 ± 2 Gyr and a metallicity of mathrm Fe/H = -0.5 ± 0.2. Our photometric and spectroscopic determinations are in excellent agreement.