The contribution of AGN to the submillimetre population

We find that X-ray sources in the Extended Chandra Deep Field-South are strongly spatially correlated with Large APEX Bolometer Camera (LABOCA) 870 mu m sources. We investigate the dependence of this correlation on X-ray flux, hardness ratio and column density, finding that specifically faint and absorbed X-ray sources are significant sub-mm emitters. In the X-ray source redshift subsample we confirm the previous result that higher luminosity sources (L-X > 1044 erg s-1) have greater 870 mu m fluxes but we also find that this subsample selects against absorbed sources, faint in X-ray flux. Overall, we find that X-ray sources contribute 1.5 +/- 0.1 Jy deg-2 to the sub-mm background, approximate to 3 per cent of the total, in agreement with the prediction of an obscured active galactic nuclei (AGN) model which also gives a reasonable fit to the bright sub-mm source counts. This non-unified model also suggests that when Compton-thick, X-ray-undetected sources are included, then the fractional AGN contribution to the sub-mm background would rise from approximate to 3 per cent to a total of 25-40 per cent, although in a unified model the AGN contribution would only reach approximate to 13 per cent, because the sub-mm flux of the X-ray sources is then more representative of the whole AGN population. Measurements of the dependence of sub-mm flux on X-ray flux, luminosity and column density all agree well with the predictions of the non-unified AGN model. Heavily absorbed, X-ray-undetected AGN could explain the further cross-correlation we find between sub-mm sources and z > 0.5 red galaxies. We conclude that sub-mm galaxies may contain the long-sought absorbed AGN population needed to explain the X-ray background.