L_7.7   μm as a function of the continuum luminosity νL_ν(5.8 μm). The red diamonds are the sources of our sample. The open (full) black triangle is the stack of our sources with EW_7.7   μm > (<)6μm. The full black circle is the composite spectrum of the SMG sample from Menéndez-Delmestre et al. (2009). The black crosses are the PAH-rich sources from Sajina et al. (2007). The black asterisks are the AGN-dominated sources from Coppin et al. (2010). The black line is the best fit of all these samples. The dashed black line is the best fit to the local starbursts from Brandl et al. (2006) (black squares).