The nature of sub-millimetre galaxies I: a comparison of AGN and star-forming galaxy SED fits

High redshift sub-millimetre galaxies (SMGs) are usually assumed to be powered by star-formation. However, it has been clear for some time that $>$20% of such sources brighter than $\approx3$mJy host quasars. Here we analyse a complete sample of 12 sub-mm LABOCA/ALMA 870 $\mu$m sources in the centre of the William Herschel Deep Field (WHDF) with multi-wavelength data available from the X-ray to the radio bands. Previously, two sources were identified as X-ray absorbed quasars at $z=1.32$ and $z=2.12$. By comparing their spectral energy distributions (SEDs) with unabsorbed quasars in the same field, we confirm that they are dust reddened although at a level significantly lower than implied by their X-ray absorption. Then we compare the SED's of all the sources to dust-reddened AGN models and to similarly reddened star-forming galaxy models, one based on the median SMG fit to the large AS2UDS survey. This optical/NIR comparison combined with Spitzer MIR colours and faint Chandra X-ray detections shows that 7/12 SMGs are best fitted with an obscured quasar model, a further 3/12 show no preference between AGN and star-forming templates, leaving only a $z=0.046$ spiral galaxy and one unidentified source. So in our complete sample, the vast majority (10/12) of bright SMGs are at least as likely to fit an AGN as a star-forming galaxy template. We then modify the model of Hill & Shanks (2011) and conclude that obscured AGN in SMGs may still provide the dominant contribution to both the hard X-ray and sub-millimetre backgrounds.