The orbital evolution of UFDs and GCs in an evolving Galactic potential
  We use the second Gaia data release to investigate the kinematics of 17
ultra-faint dwarf galaxies (UFDs) and 154 globular clusters (GCs) in the Milky
Way, focusing on the differences between static and evolving models of the
Galactic potential. An evolving potential modifies a satellite's orbit relative
to its static equivalent, though the difference is small compared to existing
uncertainties on orbital parameters. We find that the UFD Bo\"otes II is likely
on its first passage around the Milky Way. Depending on the assumed mass of the
Milky Way, the UFDs Triangulum II, Hydrus I, Coma Berenices, Draco II, and Ursa
Major II, as well as the GC Pyxis, may also be on first infall so may be useful
for constraining the mass of the Galaxy. We identify a clear kinematic
distinction between metal-rich (${\rm [Fe/H]}>-1.1$) and metal-poor GCs (${\rm
[Fe/H]}\leq-1.1$). Although most metal-rich clusters occupy predominately
prograde orbits, with low eccentricities ($e\approx 0.35$) and similar specific
angular momenta and orbital planes as the Galactic disc, 7 show potentially
retrograde orbits, the origin of which is unclear. Metal-poor clusters have
more diverse orbits, higher eccentricities ($e\approx 0.65$), and half have
orbital planes offset from the disc by 60 to 120 degrees. The UFDs have similar
$\theta$ and $\phi$ to the metal-poor GCs, suggesting a similar origin. We
provide a catalogue of orbital parameters for UFDs and GCs for two different
Galaxy masses and their observational uncertainties.
