Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope that, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (∼ 20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a > 10× increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population show enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure -2time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions.

Previous studies of the active galactic nuclei (AGNs) contribution to the cosmic X-ray background (CXB) consider only observable parameters such as luminosity and absorbing column. Here, for the first time, we extend the study of the CXB to physical parameters including the Eddington ratio of the sources and the black hole mass. In order to calculate the contribution to the CXB of AGN accreting at various Eddington ratios, an evolving Eddington ratio space density model is calculated. In particular, Compton thick (CT) AGNs are modeled as accreting at specific, physically motivated Eddington ratios instead of as a simple extension of the Compton thin type 2 AGN population. Comparing against the observed CT AGN space densities and log N -log S relation indicates that CT AGNs are likely a composite population of AGNs made up of sources accreting either at >90% or <1% of their Eddington rate.

At energies 2 keV, active galactic nuclei (AGN) are the source of the cosmic X-ray background (CXB). For AGN population synthesis models to replicate the peak region of the CXB (∼30 keV), a highly obscured and therefore nearly invisible class of AGN, known as Compton thick (CT) AGN, must be assumed to contribute nearly a third of the CXB. In order to constrain the CT fraction of AGN and the CT number density we consider several hard X-ray AGN luminosity functions and the contribution of blazars to the CXB. Following the unified scheme, the radio AGN luminosity function is relativistically beamed to create a radio blazar luminosity function. An average blazar spectral energy density model is created to transform radio luminosity to X-ray luminosity. We find the blazar contribution to the CXB to be 12% in the 0.5-2 keV band, 7.4% in the 2-10 keV band, 8.9% in the 15-55 keV band, and 100% in the MeV region. When blazars are included in CXB synthesis models, CT AGN are predicted to be roughly one-third of obscured AGN, in contrast to the prediction of one half if blazars are not considered. Our model implies a BL Lac X-ray duty cycle of ∼13%, consistent with the concept of intermittent jet activity in low power radio galaxies.

Due to the co-evolution of supermassive black holes and their host galaxies, understanding the mechanisms that trigger active galactic nuclei (AGN) are imperative to understanding galaxy evolution and the formation of massive galaxies. It is observationally difficult to determine the trigger of a given AGN due to the difference between the AGN lifetime and triggering timescales. Here, we utilize AGN population synthesis modeling to determine the importance of different AGN triggering mechanisms. An AGN population model is computed by combining an observationally motivated AGN triggering rate and a theoretical AGN light curve. The free parameters of the AGN light curve are constrained by minimizing a χ 2 test with respect to the observed AGN hard X-ray luminosity function. The observed black hole space density, AGN number counts, and Xray background spectrum are also considered as observational constraints. It is found that major mergers are not able to account for the entire AGN population. Therefore, non-merger processes, such as secular mechanisms, must also trigger AGN. Indeed, non-merger processes are the dominant AGN triggering mechanism at z 1-1.5. Furthermore, the shape and evolution of the black hole mass function of AGN triggered by major mergers is intrinsically different from the shape and evolution of the black hole mass function of AGN triggered by secular processes.