The Nuclear Star Cluster and Nuclear Stellar Disk of the Milky Way: Different Stellar Populations and Star Formation Histories

Abstract

The Milky Way’s nuclear stellar disk (NSD) and nuclear star cluster (NSC) are the main features of the Galactic center. Nevertheless, their observation is hampered by the extreme source crowding and high extinction. Hence, their relation and formation scenario are not fully clear yet. We aim to detect the stellar populations from the NSC and the NSD along the line of sight toward the NSC and assess whether they have different stellar populations and star formation histories. We analyzed the color–magnitude diagram, K s versus H − K s , of a region of 8.′2 × 2.′8 centered on the NSC, and detected two different stellar groups with different extinctions. We studied their red clumps to find the features associated with each of the stellar populations. We obtained that the two groups of stars correspond to the NSD and the NSC and found that they have significantly different stellar populations and star formation histories. We detected a double red clump for the NSD population, in agreement with previous work, whereas the NSC presents a more complex structure well fitted by three Gaussian features. We created extinction maps to analyze the extinction variation between the detected stellar groups. We found that the high-extinction layer varies on smaller scales (arcseconds) and that there is a difference of A K s ∼ 0.6 mag between both extinction layers. Finally, we obtained that the distance toward each of the stellar populations is compatible with the Galactic center distance and found some evidence of a slightly closer distance for the NSD stars (∼360 ± 200 pc).