What is a Galaxy?

What is a galaxy? How Cold is Cold Dark Matter? Recent progress in Near Field Cosmology

These data show that there is a bimodal distribution in half-light radii, with stable star clusters always being smaller than 35pc, while stable galaxies are always larger than 120pc. We extend the previously known observational relationships and interpret them in terms of a more fundamental pair of intrinsic properties of dark matter itself: dark matter forms cored mass distributions, with a core scale length of greater than about 100pc, and always has a maximum central mass density with a narrow range.
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Galaxies are embedded in dark matter halos with these properties; smaller systems containing dark matter are not observed