Origin of the Local Group satellite planes

Abstract

We attempt to understand the planes of satellite galaxies orbiting the Milky\nWay (MW) and M31 in the context of Modified Newtonian Dynamics (MOND), which\nimplies a close MW-M31 flyby occurred ${\\approx 8}$ Gyr ago. Using the timing\nargument, we obtain MW-M31 trajectories consistent with cosmological initial\nconditions and present observations. We adjust the present M31 proper motion\nwithin its uncertainty in order to simulate a range of orbital geometries and\nclosest approach distances. Treating the MW and M31 as point masses, we follow\nthe trajectories of surrounding test particle disks, thereby mapping out the\ntidal debris distribution.\n Around each galaxy, the resulting tidal debris tends to cluster around a\nparticular orbital pole. We find some models in which these preferred spin\nvectors align fairly well with those of the corresponding observed satellite\nplanes. The radial distributions of material in the simulated satellite planes\nare similar to what we observe. Around the MW, our best-fitting model yields a\nsignificant fraction (0.22) of counter-rotating material, perhaps explaining\nwhy Sculptor counter-rotates within the MW satellite plane. In contrast, our\nmodel yields no counter-rotating material around M31. This is testable with\nproper motions of M31 satellites.\n In our best model, the MW disk is thickened by the flyby 7.65 Gyr ago to a\nroot mean square height of 0.75 kpc. This is similar to the observed age and\nthickness of the Galactic thick disk. Thus, the MW thick disk may have formed\ntogether with the MW and M31 satellite planes during a past MW-M31 flyby.\n