IRS spectra of the 16 ULIRGs analysed in this work (grey lines; Desai et al. 2007; Armus et al. 2007). These mid-infrared spectra show strong PAH features in the range 6 to 17 μm, as well as a rising continuum from hot dust out to 35 μm, indicating strong star formation in these systems. The strong absorption feature at λ ≃ 10 μm is due to silicates and indicates high dust optical depths in these galaxies. For comparison, we plot the starburst template of Brandl et al. (2006) in red. All the spectra are normalised to unity at 15 μm. The strengths of the silicate absorption feature and the 6.2-μm PAH equivalent widths of all spectra are presented in Table 2.