Two-dimensional histograms of the relative frequency of occurrence of normalized Hα surface brightness values versus the projected distance for galaxies in 4 equal-size bins in galaxy number selected on their isophotal area (A_iso). The relative frequency of occurrence is scaled in the same way as in Fig. 2. From left to right, each panel represents an increasing angular size (i.e., A_iso < 1.4 arcsec², 1.4 arcsec²  ≥ A_iso < 2.5 arcsec², 2.5 arcsec²  ≥ A_iso < 3.3 arcsec² and A_iso ≥ 3.3 arcsec²). The values of r_iso, which is derived from A_iso, for individual galaxies are given in Table 1. The galaxies with the smaller dynamic range (higher surface brightness detection limits) in the data are also smaller in isophotal size.