Implications of an Obscured AGN Model for the X-ray Background at Sub-mm and Far Infra-Red Wavelengths

Abstract

Models invoking large populations of obscured AGN are known to provide good fits to the spectrum of the X-ray background and the observed soft and hard X-ray number counts. An important consequence of these models is that significant quantities of dust are required in order to provide the obscuring medium, which will be heated by the nuclear radiation from the AGN, and will radiate in the thermal infra-red. We therefore model the properties of the dust, and predict the contribution of obscured AGN to the intensity of the far infra-red background and the sub-mm source counts and redshift distribution, in order to ascertain whether our models are constrained further by the data available at these wavelengths. Our conservative models predict a contribution of between 5 and 15 per cent of the far infra-red background intensity, whereas if we use more extreme values for several parameters, this value may reach 33 per cent. This suggests that AGN may only form a significant minority of sub-mm sources, in agreement with the results of spectroscopic follow-up. Our models thus show that there is no inconsistency between obscured AGN models for the X-ray background and either sub-mm source counts or the intensity of the far infra-red background. We further propose that obscured AGN may explain the sub-mm emission associated with Extremely Red Objects (EROs). Finally, we make predictions for the redshift distribution of AGN sources detected in faint sub-mm surveys, which should allow future tests of this obscured AGN model.