Kinematics of metallicity populations in Omega Centauri using the Gaia Focused Product Release and Hubble Space Telescope

¹ Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 2, 35122 Padova, Italy
² INAF – Ossevatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

^★ Corresponding author; nagaraj.vernekar@inaf.it; nagarajbadarinarayan.vernekar@studenti.unipd.it

Received: 27 November 2024
Accepted: 24 February 2025

Abstract

Context. Omega Cen is the largest known globular cluster in the Milky Way. It is also a quite complex object with a large metallicity spread and multiple stellar populations. Despite a number of studies over the past several decades, the series of events that led to the formation of this cluster is still poorly understood. One of its peculiarities is the presence of a metal-rich population that does not show the phenomenon of light-element anti-correlations (C-N, Na-O, Mg-Al), a trait that is considered characteristic of Galactic globular clusters and present among more metal-poor Omega Cen stars. This leads to speculation that such an anomalous population was accreted by the cluster.

Aims. We aim to investigate the kinematics of Omega Cen populations to gain insight into the formation scenario of the cluster.

Methods. Using the newly released Gaia FPR and DR3 catalogue, we conducted a detailed kinematical analysis of cluster members within Omega Cen. The cluster members were divided into four metallicity populations, and their mean proper motion in radial and tangential components were compared with each other. We also performed Gaussian-mixture model fitting on the metallicity distribution to estimate the number of populations within our sample and an independent analysis of the Hubble Space Telescope catalogue as confirmation.

Results. The mean proper motions (μ_r and μ_t) of the metallicity populations do not show any significant differences. It is also not dependent on the approach chosen to determine the number of metallicity populations. We do find a clear signature of rotation in all of the populations (including the metal-rich one) with similar velocities.