Representative IRS spectra of star-forming galaxies with the PAH fit results overlaid. An averaged spectrum of nearby starburst galaxies of Brandl et al. (2006; top right) and four “noise-free galaxy spectral templates” of Smith et al. (2007; remaining four panels) are shown in black lines. The individual PAH features (blue), the power-law continuum (blue), and their sum (black) are overlaid in each panel. The residual of the fit (observed–fitted) is shown at offset baseline (horizontal broken line) for clarity.