The nature of the sources of the mid-IR background light

Deep mid-IR galaxy surveys performed with ISOCAM have revealed a population of dust-obscured sources, for which the observed source density is in large excess with respect to the Euclidean predictions, and which suffices to account for about 60% of the diffuse infrared background light (IBL) at 15 pm. Synthesis models for the hard X-ray background, which require a large population of heavily obscured AGNs, predict that a large fraction of the accretion power in the Universe is absorbed. The absorbed energy is expected to be re-emitted in the IR-mm waveband. Key current questions are: What is the nature of the sources in the IR population, and what fraction of the deep X-ray and mid-IR populations are related.