Tidal debris from Omega Centauri discovered with unsupervised machine learning

Abstract

ABSTRACT The gravitational interactions between the Milky Way and in-falling satellites offer a wealth of information about the formation and evolution of our Galaxy. In this paper, we explore the high-dimensionality of the GALAH DR3 plus Gaia eDR3 data set to identify new tidally stripped candidate stars of the nearby star cluster Omega Centauri ($\omega \, \mathrm{Cen}$). We investigate both the chemical and dynamical parameter space simultaneously, and identify cluster candidates that are spatially separated from the main cluster body, in regions where contamination by halo field stars is high. Most notably, we find candidates for $\omega \, \mathrm{Cen}$ scattered in the halo extending to more than 50° away from the main body of the cluster. Using a grid of simulated streams generated with $\omega \, \mathrm{Cen}$-like orbital properties, we then compare the on sky distribution of these candidates to the models. The results suggest that if $\omega \, \mathrm{Cen}$ had a similar initial mass as its present day mass, then we can place a lower limit on its time of accretion at tacc > 7 Gyr ago. Alternatively, if the initial stellar mass was significantly larger, as would be expected if $\omega \, \mathrm{Cen}$ is the remnant core of a dwarf Galaxy, then we can constrain the accretion time to tacc > 4 Gyr ago. Taken together, these results are consistent with the scenario that $\omega \, \mathrm{Cen}$ is the remnant core of a disrupted dwarf galaxy.