Figure 2

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 R500, where the enclosed density is 500 times the cosmic mean [284]. The detected baryon fraction fd = Mb ∕ (0.17M500 ), where Mb is the detected baryonic mass, 0.17 is the universal baryon fraction [229], and M500 is the dynamical mass (baryonic + dark mass) enclosed by R500. 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 ΛCDM.