Astronomers have estimated that VVV CL001, a faint globular cluster located near the center of the Milky Way, is approximately 13.4 billion years old. If confirmed, the result would make it one of the oldest known star clusters in the galaxy, and possibly the oldest in its central bulge.
The cluster has been difficult to study because it lies behind dense clouds of interstellar dust. Its small size and low brightness also limited earlier observations, while astronomers have not detected RR Lyrae stars in the cluster, which are commonly used to determine distances.
VVV CL001 was discovered in 2010 by the VISTA Variables in the Vía Láctea survey and confirmed as a cluster in 2019. Its apparent proximity to the galactic center, combined with a motion that differs from nearby stars, raised questions about its origin and whether it truly belongs to that region. Determining its orbit required a more precise measurement of its distance.
In the new study, led by J. Olivares Carvajal of the INAF Capodimonte Astronomical Observatory, researchers analyzed deep near-infrared observations from the FourStar camera on the Magellan Baade 6.5-meter Telescope. They also used Hubble Space Telescope images to investigate the cluster's age.
By tracking stellar motions and applying a machine-learning algorithm, the team identified about 155 likely cluster members. The sample included stars in the horizontal-branch and main-sequence-turnoff stages, which had not been clearly observed in previous studies. The main-sequence turnoff is especially useful for estimating age, while horizontal-branch stars can serve as brightness references when determining distance.
Comparison with theoretical models placed VVV CL001 at approximately 7.34 kiloparsecs, or about 24,000 light-years, from Earth. This distance puts the cluster within the Milky Way's central bulge and reportedly has the smallest error among published estimates for the object.
Using Hubble observations and the cluster's turnoff stars, the researchers obtained an age of about 13.4 billion years, with an uncertainty of 0.4 billion years. The result suggests that VVV CL001 could be the oldest globular cluster in the bulge and perhaps in the Milky Way.
The estimate remains below the universe's approximate age of 13.8 billion years, but its proximity to that limit makes further verification important. The researchers plan high-resolution spectroscopic observations to test the result through the cluster's chemical composition. The study was posted on arXiv on Oct. 20 and has been accepted for publication in Astronomy & Astrophysics.
Sources: Phys.org.