Figure 2: The fraction of the expected baryons that are detected as a function of potential-well depth
(bottom axis) and mass (top). Measurements are referenced to the radius , where the enclosed
density is 500 times the cosmic mean [284]. The detected baryon fraction ,
where is the detected baryonic mass, 0.17 is the universal baryon fraction [229], and
is the dynamical mass (baryonic + dark mass) enclosed by . Each point is a bin representing
many objects. Gray triangles represent galaxy clusters, which come close to containing the cosmic
fraction. The detected baryon fraction declines systematically for smaller systems. Dark-blue circles
represent star-dominated spiral galaxies. Light-blue circles represent gas-dominated disk galaxies.
Orange squares represent Local Group dwarf satellites for which the baryon content can be less than
1% of the cosmic value. Where these missing baryons reside is one of the challenges currently faced
by CDM.
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