Number of Hα photons as derived from the line strength plotted against the total number of available ionizing photons. As a rule of thumb, about two ionizing photons are needed to produce one Hα photon (the black line). If a point is located above this line, there are not enough ionizing photons to produce the Hα emission. Blue crosses: galaxies for which only one (old) galaxy model was needed to fit the observed SED. Green squares: galaxies with a second younger galaxy component used in the model fit. Only one blue cross (B2 1455+28) is above the line, but it is a single-component galaxy only due to the lack of UV information (see Sect. 3.1).