L_7.7   μm as a function of L_6.2   μm. The red diamonds are the sources of our sample. The open (full) black triangle is the stack of our subsample with EW_7.7   μm >   (<)   6   μm. The full black circle is the stacked spectrum of the SMG sample from Menéndez-Delmestre et al. (2009). The black circles are the intermediate-redshift star-forming galaxies from Shi et al. (2009). The black stars are the sample of SMGs from Pope et al. (2008). 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 full black squares are the sources from Farrah et al. (2008). The open black squares are the local star-forming galaxies from Brandl et al. (2006). The solid line is our best fit to all these samples. The dashed line is the best fit found by Pope et al. (2008) for their SMGs and the local starbursts.