Baryonic effects on the number density of halos. Top panel: the green line is the VF of all galaxies, as measured by ALFALFA (same as in Fig. 3). The green dashed line assumes an incompleteness of the ALFALFA VF at low velocities, parameterized as (see Sect. 4.2). The solid black line represents the VF of halos in the DM-only BolshoiP simulation (also same as in Fig. 3). The dotted black line corresponds to the HVF of halos hosting a stellar counterpart, according to the results of the hydrodynamical simulations of Sawala et al. (2014). The flattening of the hydrodynamical HVF at velocities ≲25 km s^-1 is due to the suppression of galaxy formation caused by reionization feedback (see text for details). Bottom panel: the blue dotted line is the average V_rot − V_h relation according to the hydrodynamical result. The dashed blue line additionally takes into account a possible incompleteness of the ALFALFA VF. The gray symbols represent our sample of galaxies with resolved HI rotation curves. These datapoints are slightly shifted with respect to their positions in Fig. 6, because a correction for the cosmic baryon fraction is not necessary when comparing against a hydrodynamical simulation.