Two-dimensional histogram of the frequency of occurrence of pixel values of Hα surface brightness versus projected physical distance for the entire sample. As shown in the color bar, the darkest regions have the highest frequency of occurrence. The surface brightness has not been corrected for cosmological dimming. The zero of the projected distance was chosen to be the symmetry point in either the Hα velocity field or of 3σ isophotes in the Hα surface brightness map.