The average submillimetre properties of Lyman α 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 Submillimetre Common User Bolometer Array 2 (SCUBA-2) observations of the Small Selected Area 22(h) field to determine the average 850 mu m flux density of 34 LABs. We measure S-850 = 0.6 +/- 0.2 mJy 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 +/- 0.3 mJy. Only two LABs (1 and 18) have individual SCUBA-2 > 3.5 sigma detections at a depth of 1.1 mJy beam(-1). 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.