Smooth kinematic and metallicity gradients between the Milky Way's
nuclear star cluster and nuclear stellar disc. Different components of the
same structure?
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. 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. 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. We detect kinematics 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.Comment: Submitted to A&A. 13 pages, 9 figure