The Milky Way's nuclear stellar disc: A dynamically cool and metal-rich
  component formed from the Central Molecular Zone?
  The nuclear stellar disc (NSD) is, together with the nuclear star cluster
(NSC) and the central massive black hole, one of the main components in the
central parts of our Milky Way. However, until recently, only few studies of
the stellar content of the NSD have been obtained due to extreme extinction and
stellar crowding. With a dedicated KMOS (VLT, ESO) spectroscopic survey, we
study the kinematics and global metallicities of the NSD based on the
observations of K/M giant stars. We trace radial velocities and metallicities
which were derived based on spectral indices (Na I and CO) along the NSD and
compare those with a Galactic Bulge sample of APOGEE (DR16) and data from the
NSC. We find that the metallicity distribution function and the fraction of
metal-rich and metal-poor stars in the NSD are different from the corresponding
distributions and ratios of the NSC and the Galactic Bulge. By tracing the
velocity dispersion as a function of metallicity, we clearly see that the NSD
is kinematically cool and that the velocity dispersion decreases with
increasing metallicity contrary to the inner Bulge sample of APOGEE ($\rm |b| <
4^{o}$). Using molecular gas tracers ($\rm H_{2}CO$, CO(4-3)) of the Central
Molecular Zone (CMZ) we find an astonishing agreement between the gas rotation
and the rotation of the metal-rich population indicating that the metal-rich
stars could have formed from gas in the CMZ. On the other hand, the metal-poor
stars show a much slower rotation profile with signs of counter-rotation
indicating a different origin of these stars. Coupling kinematics with global
metallicities, our results demonstrate that the NSD is chemically and
kinematically distinct with respect to the inner Bulge indicating a different
formation scenario.
