V_rot − V_h relation in a ΛCDM universe. The blue line is the average relation between the galactic HI rotational velocity (V_rot) and the maximum circular velocity of the host halo (V_h), in a ΛCDM universe. This relation is obtained by abundance matching (AM), i.e., by matching the cumulative velocity distributions of all galaxies and “galactic” halos (see Fig. 3). The dashed blue line is the AM result when galaxies are matched to all halos. The dashed black line is a reference one-to-one line.