HST Proper Motions of NGC 147 and NGC 185: Orbital Histories and Test of
  Dynamically Coherent Andromeda Satellite Plane
  We present the first proper motion (PM) measurements for the dwarf elliptical
galaxies NGC 147 and NGC 185, two satellite galaxies of M31, using multi-epoch
HST imaging data with time baselines of $\sim 8$ years. For each galaxy, we
take an error-weighted average of measurements from HST ACS/WFC and WFC3/UVIS
to determine the PMs. Our final results for the PMs are $(\mu_\mathrm{W},
\mu_\mathrm{N})_\mathrm{N147} = (-0.0232, 0.0378) \pm (0.0143,
0.0146)\,\mathrm{mas}\,\mathrm{yr}^{-1}$ for NGC 147, and $(\mu_\mathrm{W},
\mu_\mathrm{N})_\mathrm{N185} = (-0.0242, 0.0058) \pm (0.0141,
0.0147)\,\mathrm{mas}\,\mathrm{yr}^{-1}$ for NGC 185. The 2-dimensional
direction of motion for NGC 147 about M31 is found to be aligned with its tidal
tails. The 3-d positions and velocities of both galaxies are transformed into a
common M31-centric coordinate system to study the detailed orbital histories of
the combined M31+NGC 147+NGC 185 system via numerical orbit integration. We
find that NGC 147 (NGC 185) had its closest passage to M31 0.3-0.5~Gyr
($\gtrsim 1.6$~Gyr) within the past 6 Gyr at distances of $\sim 70$ kpc (70-260
kpc). The pericentric times of NGC 147/NGC 185 correlate qualitatively well
with the presence/absence of tidal tails seen around the galaxies. Our PMs show
that the orbital poles of NGC 147, and also NGC 185 albeit to a lesser degree,
agree within the uncertainties with the normal of the Great Plane of Andromeda
(GPoA). These are the first measurements of the 3-d angular momentum vector of
any satellite identified as original GPoA members. Our results strengthen the
hypothesis that the GPoA may be a dynamically coherent entity. We revisit
previous claims that NGC 147 and NGC 185 are binary galaxies and conclude that
it is very unlikely the two galaxies were ever gravitationally bound to each
other.
