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

Abstract

The Galactic centre (GC) is located at only 8 kpc from Earth and constitutes a unique template 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). 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. We use photometric, kinematic, and metallicity data to distinguish between probable NSD and NSC stars in a region centred on the NSC. We detected two different extinction groups of stars and obtained significantly different proper motion distributions for each of them, in agreement with the expected kinematics for the NSD and the NSC. We derived radial velocity maps that appear to be different for the NSD and the NSC. We also found different metallicities for each of the components, 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). 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, $[M/H]\sim 0.3$\,dex, obtained for the NSC metal rich stars, supports that the NSC is arguabily the most metal rich region of the Galaxy.

Accepted for publication in A&A. 8 pages, 8 figures

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• The Star Formation History of the Milky Way's Nuclear Star Cluster
• The formation history of our Galaxy's nuclear stellar disc constrained from HST observations of the Quintuplet field
• Evidence of an age gradient along the line of sight in the nuclear stellar disc of the Milky Way
• GD-1 Stellar Stream and Cocoon in the DESI Early Data Release
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• A first glimpse at the line-of-sight structure of the Milky Way's nuclear stellar disc
• Smooth kinematic and metallicity gradients reveal that the Milky Way's nuclear star cluster and disc might be part of the same structure
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• Age and metallicity of the Milky Way's nuclear star cluster studied at 3 pc from Sagittarius A*
• Chemical analysis of the Milky Way's Nuclear Star Cluster: Evidence for a metallicity gradient