The stacked HI 21 cm emission spectra (in luminosity density) for blue galaxies in the three M _B bins, [A] − 16.0 ≥ M _B > − 17.6, [B] −17.6 ≥ M _B > −19.2, and [C] − 19.2 ≥ M _B ≥ − 22.0. The dashed black curves in each panel show the rms noise in the corresponding velocity planes of the stacked spectral cubes. See text for details.