Incompleteness of the ALFALFA survey at low velocities. Top panel: the blue dotted, dashed and dash-dotted lines represent three relations for the “HI-selection” factor, f_HI(V_rot), each accounting for progressively more low-velocity systems that are undetectable by ALFALFA due to their low HI mass (Eqs. (9)–(11)). Bottom panel: the blue dotted, dashed and dash-dotted lines are the average V_rot − V_h relations for the corresponding f_HI(V_rot) relations shown in the top panel. For reference, we also plot our fiducial relation with a thick solid blue line (same as in Fig. 4). The gray symbols represent our sample of galaxies with HI rotation curves, same as in Fig. 6.