Top: ionization parameter computed for co-eval stellar populations as a function of time and for constant star formation, taken after 500 Myr. We show the predictions for stellar populations containing only single stars in gray shades and the model when stripped stars are included in purple shades. For constant star formation, the contribution from stripped stars is about 2% and therefore we do not show the markers for single star populations as they overlap. We show measurements of the ionization parameter for groups of observed galaxies to the right of the diagram (Moustakas et al. 2010; Nakajima & Ouchi 2014). Bottom: table explaining which gas density and stellar mass for co-eval stellar populations that correspond to which contour in the diagram using numbers as labels. In the case of constant star formation, the numbers are correlated with combinations of the gas density and star formation rate instead.