Figure 38: The characteristic acceleration, in units of , 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
and . 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).
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