IAUS379: Dynamical Masses of Local Group Galaxies

The satellite galaxy systems of the Milky Way (MW) and M31 both show a very thin kinematically coherent structure or satellite plane. It has previously been shown that each plane is in 3.55σ tension with ΛCDM expectations, which combined with the similar structure around Centaurus A falsifies the model at 5.3σ confidence. In this talk, I will present hydrodynamical simulations of the MW and M31 in Milgromian dynamics (MOND), which requires them to have experienced a past close flyby roughly 9 Gyr ago. While the formation of tidal dwarf galaxies is not resolved, the tidal debris around each galaxy ends up distributed anisotropically. In each case, the preferred orientation matches that of the actually observed satellite galaxy plane (MNRAS, 513, 129). The MW and M31 retain thin discs with realistic scale lengths, orientations, and present separation. I will therefore argue that the Local Group satellite planes are best understood as arising from a past close MW-M31 flyby in MOND, whose enhancement to gravity at low accelerations naturally explains the apparent dark matter content of the satellite plane members. This contrasts with the Newtonian picture where tidal dwarfs should lack dark matter, while primordial dwarfs should be distributed almost isotropically and should moreover constitute the dominant satellite population.