Smooth kinematic and metallicity gradients reveal that the Milky Way’s nuclear star cluster and disc might be part of the same structure

Abstract

Context. The innermost regions of most galaxies are characterised by the presence of extremely dense nuclear star clusters. Nevertheless, these clusters are not the only stellar component present in galactic nuclei, where larger stellar structures known as nuclear stellar discs, have also been found. Understanding the relation between nuclear star clusters and nuclear stellar discs is challenging due to the large distance towards other galaxies which limits their analysis to integrated light. The Milky Way’s centre, at only ∼8 kpc, hosts a nuclear star cluster and a nuclear stellar disc, constituting a unique template to understand their relation and formation scenario. Aims. We aim to study the kinematics and stellar metallicity of stars from the Milky Way’s nuclear star cluster and disc to shed light on the relation between these two Galactic centre components. Methods. We used publicly available photometric, proper motions, and spectroscopic catalogues to analyse a region of ∼2.8′×4.9′ centred on the Milky Way’s nuclear star cluster. We built colour magnitude diagrams, and applied colour cuts to analyse the kinematic and metallicity distributions of Milky Way’s nuclear star cluster and disc stars with different extinction, along the line of sight. Results. We detect kinematic and metallicity gradients for the analysed stars along the line of sight towards the Milky Way’s nuclear star cluster, suggesting a smooth transition between the nuclear stellar disc and cluster. We also find a bi-modal metallicity distribution for all the analysed colour bins, which is compatible with previous work on the bulk population of the nuclear stellar disc and cluster. Our results suggest that these two Galactic centre components might be part of the same structure with the Milky Way’s nuclear stellar disc being the grown edge of the nuclear star cluster.