Active galactic nuclei unification and the X-ray background

Active galactic nuclei (AGNs) unification, which implies a large number of obscured AGNs, can explain the optical, infrared, and X-ray content of deep multiwavelength surveys. Here we show that the same model also successfully explains the spectral shape and intensity of the X-ray background. The simplest possible unified model assumes a constant ratio of obscured to unobscured AGNs, independent of redshift or luminosity. With almost no free parameters, the predicted X-ray background agrees remarkably well with observations. Both the observed properties of deep AGN samples and the X-ray background can be explained by the same model without invoking a different evolution for obscured and unobscured AGNs. Recent observational evidence shows that the ratio of obscured to unobscured AGNs may depend on luminosity. An AGN unification model that allows for such a dependence also fits the X-ray background spectrum, and the predicted number counts in the 2 - 10 keV X-ray band are in good agreement with observations. We present predictions for the source counts in hard X-rays (10 - 100 keV), which indicate that future missions such as the Black Hole Finder probe will observe thousands of heavily obscured AGNs.