Rest-frame flux ratios of F_{rest 3.5 μm}/ F_{rest 2.0 μm} (left), F_{rest 7.7 μm}/F_{rest 3.5 μm} (middle), and F_{rest 11.3 μm}/F_{rest 7.7 μm} (right) as a function of redshift. The flux ratios of the PAH galaxies are shown with diamonds with one-sigma error bars, and ones of the templates and the models are shown with connected crosses. To enable easier visual comparison among the panels, all three kinds of the panels cover the same 1.6 dex-wide range in the flux ratios. Top: SWIRE SED templates of Sc (blue), M 82 (green), and NGC 6090 (red). Middle: SBURT models of (t_burst, Θ) = (400 Myr, 5.0), (200 Myr, 5.0), and (200 Myr, 2.0) in green, magenta, and blue, respectively. Bottom: composite model. 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).