On the contribution of active Galactic nuclei to the cosmic background radiation

We present the results of calculations of the cosmic AGN background spectrum from 3 keV (4x10(-4) mu m) to 1000 mu m. These computations make use of the measured X-ray luminosity function and its evolution, as well as fits from synthesis models of the cosmic X-ray background (CXB), to predict the AGN contribution to the cosmic infrared background (CIRB) for different models of the location and distribution of the absorbing material. By comparing our results to observational constraints, we conclude that the current deep Spitzer surveys can account for the entire AGN contribution to the CIRB at 24 mu m, but these AGNs are almost all Compton-thin. In fact, the calculations show that Compton-thick AGNs are a small fraction of the CIRB for lambda < 100 mu m. For this reason, the most efficient method of identifying the Compton-thick AGN population is through hard X-ray imaging at energies greater than or similar to 40 keV. Selection of AGNs based on their rest-frame near-IR colors will miss low-luminosity type 2 AGNs, due to contamination from the host galaxy. Finally, the AGNs that dominate the CXB must have star formation rates < 100M(circle dot) yr(-1), consistent with them having properties similar to the sources that dominate the CIRB at z similar to 1. Including the estimated reradiated emission from star formation, AGNs and their host galaxies may contribute similar to 30% of the CIRB at 70 mu m, dropping to similar to 10% at 24 mu m and similar to 1% at 1-10 mu m.