The bulk of the cosmic infrared background resolved by ISOCAM

Deep extragalactic surveys with ISOCAM revealed the presence of a large density of faint mid-infrared (MIR) sources. We have computed the 15 mu m integrated galaxy light produced by these galaxies above a sensitivity limit of 50 mu Jy. It sets a lower limit to the 15 mu m extragalactic background light of (2.4 +/- 0.5) nW m^-2 sr^-1. The redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). It peaks around z ~ 0.8 in agreement with studies in other fields. The rest-frame 15 mu m and bolometric infrared (8-1000 mu m) luminosities of ISOCAM galaxies are computed using the correlations that we establish between the 6.75, 12, 15 mu m and infrared (IR) luminosities of local galaxies. The resulting IR luminosities were double-checked using radio (1.4 GHz) flux densities from the ultra-deep VLA and WSRT surveys of the HDFN on a sample of 24 galaxies as well as on a sample of 109 local galaxies in common between ISOCAM and the NRAO VLA Sky Survey (NVSS). This comparison shows for the first time that MIR and radio luminosities correlate up to z ~ 1. This result validates the bolometric IR luminosities derived from MIR luminosities unless both the radio-far infrared (FIR) and the MIR-FIR correlations become invalid around z ~ 1. The fraction of IR light produced by active nuclei was computed from the cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively. We find that at most 20% of the 15 mu m integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. About 75% of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (L_IR >= 10¹¹ L_sun). They exhibit star formation rates of the order of ~ 100 M_sun yr^-1. The comoving density of infrared light due to these luminous IR galaxies was more than 40 times larger at z ~ 1 than today. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140 mu m was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We find that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e. (16 +/- 5) nW m^-2 sr^-1 as compared to the (25 +/- 7) nW m^-2 sr^-1 measured with the COBE satellite, with less than 10% due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.