Emissivity index β gradients with galactocentric distance R/R_opt. The upper-left panel shows all galaxies together, while individual galaxies are shown in the remaining panels. The upper-right panel illustrates examples of negative radial gradients, the lower-left rising then falling, and the lower-right panel positive radial gradients. In all panels, as in Fig. 8, points are coded by their oxygen abundance, with filled symbols corresponding to R/R_opt≤ 0.8 and open ones to larger radii.