On the Co-orbitation of Satellite Galaxies along the Great Plane of Andromeda: NGC 147, NGC 185, and Expectations from Cosmological Simulations

Abstract Half of the satellite galaxies Andromeda form a narrow plane termed Great Plane (GPoA), and their line-of-sight velocities display correlation reminiscent rotating structure. Recently reported first proper-motion measurements for on-plane satellites NGC 147 185 indicate that they indeed co-orbit along GPoA. This provides novel opportunity to compare M31 system ΛCDM expectations. We perform detailed comparison orbital alignment two beyond Milky Way with several hydrodynamical dark-matter-only cosmological simulations (Illustris TNG50, TNG100, ELVIS, PhatELVIS) in context Planes Satellite Galaxies Problem. In line previous works, we find spatial flattening velocity are already substantial tension ΛCDM, none simulated analogs simultaneously reproducing both parameters. Almost (3%–4%) systems contain poles as well aligned indicated by most likely proper motions 185. However, within current measurement uncertainties, it is common (≈70%) best-aligned consistent alignment. Yet, chance any have well-aligned observed low (≈4%). conclude confirmation tight these objects via improved measurements, or discovery similar alignments additional GPoA members, holds potential further raise