On the Co-Orbitation of Satellite Galaxies Along the Great Plane of
  Andromeda: NGC 147, NGC 185, and Expectations from Cosmological Simulations
  Half of the satellite galaxies of Andromeda form a narrow plane termed the
Great Plane of Andromeda (GPoA), and their line-of-sight velocities display
correlation reminiscent of a rotating structure. Recently reported first proper
motion measurements for the on-plane satellites NGC 147 and NGC 185 indicate
that they indeed co-orbit along the GPoA. This provides a novel opportunity to
compare the M31 satellite system to $\Lambda$CDM expectations. We perform the
first detailed comparison of the orbital alignment of two satellite galaxies
beyond the Milky Way with several hydrodynamical and dark-matter-only
cosmological simulations (Illustris TNG-50, TNG-100, ELVIS, PhatELVIS), in the
context of the Planes of Satellite Galaxies Problem. In line with previous
works, we find that the spatial flattening and line-of-sight velocity
correlation alone is already in substantial tension with $\Lambda$CDM, with
none of the simulated analogs simultaneously reproducing both parameters.
Almost none (3 to 4\%) of the simulated systems contain two satellites with
orbital poles as well aligned with their satellite plane as indicated by the
most-likely proper motions of NGC 147 and NGC 185. However, within current
measurement uncertainties it is common (~70%) that the two best-aligned
satellites of simulated systems are consistent with the orbital alignment. Yet,
the chance that any two simulated on-plane satellites have as well aligned
orbital poles as observed is low (~4%). We conclude that confirmation of the
tight orbital alignment for these two objects via improved measurements, or the
discovery of similar alignments for additional GPoA members, hold the potential
to further raise the tension with $\Lambda$CDM expectations.
