On the Contribution of Active Galactic Nuclei to the Cosmic Background Radiation

We present the results of calculations of the cosmic AGN background spectrumfrom 3 keV (4\times 10^{-4} microns) to 1000 microns. These computations makeuse of the measured X-ray luminosity function and its evolution, as well asfits from synthesis models of the cosmic X-ray background (CXB) to predict theAGN contribution to the cosmic infrared background (CIRB) for different modelsof the location and distribution of the absorbing material. By comparing ourresults to observational constraints we conclude that the current deep Spitzersurveys can account for the entire AGN contribution to the CIRB at 24 microns,but these AGN are almost all Compton-thin. In fact, the calculations show thatCompton-thick AGN are a small fraction of the CIRB for \lambda < 100 microns.For this reason, the most efficient method of identifying the Compton-thick AGNpopulation is through hard X-ray imaging at energies >~ 40 keV. Selection ofAGN based on their rest-frame near-IR colors will miss low luminosity type 2AGN due to contamination from the host galaxy. Finally, the AGN that dominatethe CXB must have star formation rates < 100 M_{sun} yr^{-1}, consistent withthem having similar properties as the sources which dominate the CIRB at z ~1.Including the estimated re-radiated emission from star formation, AGN and theirhost galaxies may contribute ~30% of the CIRB at 70 microns, dropping to ~10%at 24 microns and ~1% at 1-10 microns.Comment: 7 pages, 4 figures, 3 in colour, accepted by Ap