Kinematic data rebuild the nuclear star cluster as the most metal-rich region of the Galaxy

Abstract

Context.The Galactic centre (GC) is located at only 8 kpc from Earth and constitutes a unique template with which to understand Galactic nuclei. Nevertheless, the high crowding and extinction towards the GC hamper the study of its main stellar components, the nuclear stellar disc (NSD) and the nuclear star cluster (NSC).Aims.Recent work has suggested that the NSD and the NSC can be distinguished along the line of sight towards the NSC via the different extinction of their stars. This motivated us to analyse the proper motion, radial velocity, and the metallicity distributions of the different extinction groups.Methods.We use photometric, kinematic, and metallicity data to distinguish between probable NSD and NSC stars in a region centred on the NSC.Results.We detected two different extinction groups of stars and obtained that they have significantly different proper motion distributions, in agreement with the expected kinematics for the NSD and the NSC. We also derived radial velocity maps that appear to be different for the two components, and found different metallicities for each of them, with the largest one measured for the most extinguished group of stars. We obtained that the metallicity distribution of each extinction group is best fitted by a bimodal distribution, indicating the presence of two metallicity components for each of them (a broad one slightly below solar metallicity, and a more metal-rich, narrower one that is largest for the high-extinction group of stars).Conclusions.We conclude that both extinction groups are distinct GC components with different kinematics and metallicity, and correspond to the NSD and the NSC. Therefore, it is possible to distinguish them via their different extinction. The high mean metallicity, [M/H] ∼ 0.3 dex, obtained for the metal-rich stars of the NSC supports the proposition that this component is arguably the most metal-rich region of the Galaxy.