Stellar Populations in the Transition Region of Nuclear Star Cluster and Nuclear Stellar Disc

TL;DR

This study investigates the stellar populations in the transition region between the Milky Way's nuclear star cluster and nuclear stellar disc, revealing distinct metallicity distributions that suggest different formation histories.

Contribution

It provides the first detailed spectroscopic analysis of stars in the transition region, showing a metallicity gradient and evidence for separate origins of the NSC and NSD.

Findings

Metallicity decreases from NSC to NSD

Presence of young stars aged 4-6 Myr

Distinct stellar populations in the transition region

Abstract

The Milky Way nuclear star cluster (NSC) is located within the nuclear stellar disc (NSD) in the Galactic centre. It is not fully understood if the formation and evolution of these two components are connected, and how they influence each other. We study the stellar populations in the transition region of NSC and NSD. We observed two $\sim$4.3 pc$^2$ fields with the integral-field spectrograph KMOS (VLT), located at r$\sim$20 pc (> 4 R$_e$) to the Galactic East and West of the NSC. We extract and analyse medium-resolution stellar spectra of >200 stars per field. The data contain in total nine young star candidates. We use stellar photometry to estimate the stellar masses, effective temperatures, and spectral types of the young stars. The stars are consistent with an age of 4-6 Myr, they may have formed inside the Quintuplet cluster, but were dispersed in dynamical interactions. Most stars in the two fields are red giant stars, and we measure their stellar metallicities [M/H] using full spectral fitting. We compare our [M/H] distributions to the NSC and NSD, using data from the literature, and find that the overall metallicity decreases from the central NSC, over the transition region, to the NSD. The steep decrease of [M/H] from the NSC to the region dominated by the NSD indicates that the two components have distinct stellar populations and formation histories.