Figure 3

Figure 3: The Baryonic Tully–Fisher (mass–rotation velocity) relation for galaxies with well-measured outer velocities Vf. The baryonic mass is the combination of observed stars and gas: Mb = M ∗ + Mg. Galaxies have been selected that have well observed, extended rotation curves from 21 cm interferrometric observations providing a good measure of the outer, flat rotation velocity. The dark blue points are galaxies with M ∗ > Mg [272]. The light blue points have M ∗ < Mg [276] and are generally less precise in velocity, but more accurate in terms of the harmlessness on the result of possible systematics on the stellar mass-to-light ratio. For a detailed discussion of the stellar mass-to-light ratios used here, see [272, 276]. The dotted line has slope 4 corresponding to a constant acceleration parameter, 1.2 × 10−10 m s−2. The dashed line has slope 3 as expected in ΛCDM with the normalization expected if all of the baryons associated with dark matter halos are detected. The difference between these two lines is the origin of the variation in the detected baryon fraction in Figure 2.