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

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 that could be explained by active galactic nuclei (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 10-20 per cent of the 850-mu m SCUBA sources at 1 mJy 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 were highly obscured (Compton-thick) at X-ray wavelengths.