The Proper Motion of Pyxis: the first use of Adaptive Optics in tandem
  with HST on a faint halo object
  We present a proper motion measurement for the halo globular cluster Pyxis,
using HST/ACS data as the first epoch, and GeMS/GSAOI Adaptive Optics data as
the second, separated by a baseline of about 5 years. This is both the first
measurement of the proper motion of Pyxis and the first calibration and use of
Multi-Conjugate Adaptive Optics data to measure an absolute proper motion for a
faint, distant halo object. Consequently, we present our analysis of the
Adaptive Optics data in detail. We obtain a proper motion of mu_alpha
cos(delta)=1.09+/-0.31 mas/yr and mu_delta=0.68+/-0.29 mas/yr. From the proper
motion and the line-of-sight velocity we find the orbit of Pyxis is rather
eccentric with its apocenter at more than 100 kpc and its pericenter at about
30 kpc. We also investigate two literature-proposed associations for Pyxis with
the recently discovered ATLAS stream and the Magellanic system. Combining our
measurements with dynamical modeling and cosmological numerical simulations we
find it unlikely Pyxis is associated with either system. We examine other Milky
Way satellites for possible association using the orbit, eccentricity,
metallicity, and age as constraints and find no likely matches in satellites
down to the mass of Leo II. We propose that Pyxis probably originated in an
unknown galaxy, which today is fully disrupted. Assuming that Pyxis is bound
and not on a first approach, we derive a 68% lower limit on the mass of the
Milky Way of 0.95*10^12 M_sun.
