Log([S II]/Hα) in the DIG as a function of log() for different radial bins in individual galaxies in the PHANGS–MUSE sample. Galaxies are ordered by stellar mass from lowest (top left) to highest (bottom right). Several galaxies (NGC 3351, NGC 1300, NGC 1512, NGC 3627, and NGC 1433) have higher values of this (and other) line ratios in their central radial bin (in red). Aside from this feature, the galaxies in the sample follow a remarkably tight relation between log([S II]/Hα) and Σ_Hα.