The average submillimetre properties of Lyman-alpha Blobs at z=3
  Ly-alpha blobs (LABs) offer insight into the complex interface between
galaxies and their circumgalactic medium. Whilst some LABs have been found to
contain luminous star-forming galaxies and active galactic nuclei that could
potentially power the Ly-alpha emission, others appear not to be associated
with obvious luminous galaxy counterparts. It has been speculated that LABs may
be powered by cold gas streaming on to a central galaxy, providing an
opportunity to directly observe the `cold accretion' mode of galaxy growth.
Star-forming galaxies in LABs could be dust obscured and therefore detectable
only at longer wavelengths. We stack deep SCUBA-2 observations of the SSA22
field to determine the average 850um flux density of 34 LABs. We measure S_850
= 0.6 +/- 0.2mJy for all LABs, but stacking the LABs by size indicates that
only the largest third (area > 1794 kpc^2) have a mean detection, at 4.5 sigma,
with S_850 = 1.4 +/- 0.3mJy. Only two LABs (1 and 18) have individual SCUBA-2 >
3.5 sigma detections at a depth of 1.1mJy/beam. We consider two possible
mechanisms for powering the LABs and find that central star formation is likely
to dominate the emission of Ly-alpha, with cold accretion playing a secondary
role.
