The cosmic infrared background resolved by Spitzer -: Contributions of mid-infrared galaxies to the far-infrared background

Aims. We quantify the contributions of 24 mu m galaxies to the Far-Infrared ( FIR) Background at 70 and 160 mu m. We provide new estimates of the Cosmic Infrared Background ( CIB), and compare it with the Cosmic Optical Background ( COB). Methods. Using Spitzer data at 24, 70 and 160 mu m in three deep fields, we stacked more than 19000 MIPS 24 mu m sources with S-24 >= 60 mu Jy at 70 and 160 mu m, and measured the resulting FIR flux densities. Results. This method allows a gain up to one order of magnitude in depth in the FIR. We find that the Mid-Infrared ( MIR) 24 mu m selected sources contribute to more than 70% of the Cosmic Infrared Background ( CIB) at 70 and 160 mu m. This is the first direct measurement of the contribution of MIR-selected galaxies to the FIR CIB. Galaxies contributing the most to the total CIB are thus z similar to 1 luminous infrared galaxies, which have intermediate stellar masses. We estimate that the CIB will be resolved at 0.9 mJy at 70 and 3 mJy at 160 mu m. By combining the extrapolation of the 24 mu m source counts below analysis, we obtain lower limits of 7.1 +/- 1.0 and 13.4 +/- 1.7 nW m(-2) sr(-1) for the CIB at 70 and 160 mu m, respectively. Conclusions. The MIPS surveys have resolved more than three quarters of the MIR and FIR CIB. By carefully integrating the Extragalactic Background Light ( EBL) SED, we also find that the CIB has the same brightness as the COB, around 24 nW m(-2) sr(-1). The EBL is produced on average by 115 infrared photons for one visible photon. Finally, the galaxy formation and evolution processes emitted a brightness equivalent to 5% of the primordial electromagnetic background ( CMB).