Rest-frame flux ratio diagrams between F_{rest 7.7 μm}/F_{rest 3.5 μm} and F_{rest 3.5 μm}/F_{rest 2.0 μm} (left) and between F_{rest 7.7 μm}/F_{rest 3.5 μm} and F_{rest 11.3 μm}/F_{rest 7.7 μm} (right). The flux ratios of the observations are shown with diamonds, and ones of the templates and the models are shown with connected crosses. Top: PAH galaxies in the nearby, mid-z, and higher-z redshift bins in blue, black, and red, respectively, with one-sigma error bars. Middle: SWIRE SED templates of Sc (blue), M 82 (green), and NGC 6090 (magenta) at z = 0.0–0.5 in steps of 0.05. Bottom: SBURT models of (t_burst, Θ) = (400 Myr, 5.0), (200 Myr, 5.0), and (200 Myr, 2.0), and the composite model in green, magenta, blue, and black, respectively. We caution that small changes of the flux ratios of the templates and the models are not to predict real changes of the ratios as a function of redshift, but to demonstrate an accuracy of our analysis method (Sect. 3.4).