A proper motion catalogue for the Milky Way's nuclear stellar disc
  We present the results of a large-scale proper motion study of the central
~36'x16' of the Milky Way, based on our high angular resolution GALACTICNUCLEUS
survey (epoch 2015) combined with the HST Paschen-alpha survey (epoch 2008).
Our catalogue contains roughly 80,000 stars, an unprecedented kinematic data
set for this region. We describe the data analysis and the preparation of the
proper motion catalogue. We verify the catalogue by comparing our results with
measurements from previous work and data. We provide a preliminary analysis of
the kinematics of the studied region. Foreground stars in the Galactic Disc can
be easily identified via their small reddening. Consistent with previous work
and with our expectations, we find that stars in the nuclear stellar disc have
a smaller velocity dispersion than Bulge stars, in particular in the direction
perpendicular to the Galactic Plane. The rotation of the nuclear stellar disc
can be clearly seen in the proper motions parallel to the Galactic Plane. Stars
on the near side of the nuclear stellar disc are less reddened than stars on
its far side. Proper motions enable us to detect co-moving groups of stars that
may be associated with young clusters dissolving in the Galactic Centre that
are difficult to detect by other means. We demonstrate a technique based on a
density clustering algorithm that can be used to find such groups of stars.
