Figure 38

Figure 38: The characteristic acceleration, in units of a 0, in the smallest galaxies known: the dwarf satellites of the Milky Way (orange squares) and M31 (pink squares) [285]. The classical dwarfs, with thousands of velocity measurements of individual stars [477], are largely consistent with MOND. The more recently discovered “ultrafaint” dwarfs, tiny systems with only a handful of stars [427], typically are not, in the sense that their measured velocity dispersions and accelerations are too high. This could be due to systematic uncertainties in the data [230], as we must distinguish between −1 σ ≈ 2 km s and − 1 σ ≈ 5 km s. Nevertheless, there may be a good physical reason for the non-compliance of the ultrafaint galaxies in the context of MOND. The deviation of these objects only occurs in systems where the stars are close to filling their MONDian tidal radii: the left panel shows the half light radius relative to the tidal radius. Such systems may not be in equilibrium. Brada & Milgrom [78] note that systems will no longer respond adiabatically to the influence of their host galaxy when a star in a satellite galaxy can complete only a few orbits for every orbit the satellite makes about its host. The deviant dwarfs are in this regime (right panel).