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 and star-forming galaxy models. 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 quasarmodel, 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 majority (10/12) of bright SMGs are at least as likely to fit an AGN as a
star-forming galaxy template, although no claim is made to rule out the latter
as SMG power sources. We then suggest modifications to a previous SMG number
count model and conclude that obscured AGN in SMGs may still provide the
dominant contribution to both the hard X-ray and sub-millimetre backgrounds.
