The AGN contribution to deep submillimetre surveys and the far-infrared background

Abstract

A great deal of interest has been generated recently by the results of deep submillimetre surveys, which in principle allow an unobscured view of dust-enshrouded star formation at high redshift. The extragalactic far-infrared and submillimetre backgrounds have also been detected, providing further constraints on the history of star formation. In this paper we estimate the fraction of these backgrounds and source counts which could be explained by AGN. The relative fractions of obscured and unobscured objects are constrained by the requirement that they fit the spectrum of the cosmic X-ray background. On the assumption that the spectral energy distributions of high redshift AGN are similar to those observed locally, we find that one can explain at least 10-20 per cent of the 850 micron SCUBA sources at 1mJy and a similar fraction of the far-infrared/submillimetre background. The exact contribution depends on the assumed cosmology and the space density of AGN at high redshift (z>3), but we conclude that active nuclei will be present in a significant (though not dominant) fraction of the faint SCUBA sources. This fraction could be significantly higher if a large population of AGN are highly obscured (Compton-thick) at X-ray wavelengths.