S₁₀₀/S₂₄ as a function of redshift for the whole GOODS-H sample (circles) and lower limits for the 24   μm dropouts (arrows). Both samples are colour coded based on their L_IR. Sources with a black cross are AGNs based on their X-ray emission. Filled symbols denote sources in the GOODS-H sample with spectroscopic redshift, while open symbols are sources with photometric redshift. Similarly, black circles on top of the arrows indicate that a spectroscopic redshift is available for that MIPS dropout source. Solid lines correspond to different observed SEDs of local LIRGs/ULIRGs (see Fig. 5) and horizontal black dashed line to S₁₀₀/S₂₄ = 43. The pop panel shows the SED of Arp220 at various redshifts along with the MIPS 24- and PACS 100   μm bands.