A submillimetre-bright z ~ 3 overdensity behind a z ~ 1 supercluster revealed by SCUBA-2 and Herschel

We present a wide-field (30 arcmin diameter) 850 μmSubmillimetre Common-User Bolometer Array-2 map of the spectacular three-component merging supercluster, RCS 231953+00, at z = 0.9. The brightest submillimetre galaxy (SMG) in the field (S₈₅₀ ≈ 12 mJy) is within 30 arcsec of one of the cluster cores (RCS 2319-C), and is likely to be a more distant, lensed galaxy. Interestingly, the wider field around RCS 2319-C reveals a local overdensity of SMGs, exceeding the average source density by a factor of 4.5, with a <1 per cent chance of being found in a random field. Utilizing Herschel observations from the Spectral and Photometric Imaging Receiver, we find that three of these SMGs have similar submillimetre colours. We fit their observed 250-850 μm spectral energy distributions to estimate their redshift, yielding 2.5<z<3.5, and calculate prodigious star formation rates ranging from 500 to 2500M⊙ yr^-1. We speculate that these galaxies are either lensed SMGs, or signpost a physical structure at z ≈ 3: a 'protocluster' inhabited by young galaxies in a rapid phase of growth, destined to form the core of a massive galaxy cluster by z = 0.