Ultra-red Galaxies Signpost Candidate Protoclusters at High Redshift

We present images obtained with LABOCA of a sample of 22 galaxies selected via their red Herschel SPIRE colors. We aim to see if these luminous, rare, and distant galaxies are signposting dense regions in the early universe. Our 870 mu m survey covers an area of approximate to 1 deg(2) down to an average rms of 3.9 mJy beam(-1), with our five deepest maps going approximate to 2x deeper still. We catalog 86 dusty star-forming galaxies (DSFGs) around our "signposts," detected above a significance of 3.5 sigma. This implies a 100(-30)(+30)% overdensity of S-870 > 8.5 mJy (or L-FIR = 6.7 x 10(12)-2.9 x 10(13) L-circle dot) DSFGs, excluding our signposts, when comparing our number counts to those in "blank fields." Thus, we are 99.93% confident that our signposts are pinpointing overdense regions in the universe, and approximate to 95% [50%] confident that these regions are overdense by a factor of at least >= 1.5 x [2x]. Using template spectral energy distributions (SEDs) and SPIRE/LABOCA photometry, we derive a median photometric redshift of z = 3.2 +/- 0.2 for our signposts, with an inter-quartile range of z = 2.8-3.6, somewhat higher than expected for similar to 850 mu m selected galaxies. We constrain the DSFGs that are likely responsible for this overdensity to within vertical bar Delta(z)vertical bar <= 0.65 of their respective signposts. These "associated" DSFGs are radially distributed within (physical) distances of 1.6 +/- 0.5 Mpc from their signposts, have median star formation rates (SFRs) of approximate to(1.0 +/- 0.2) x 10(3) M-circle dot yr(-1) (for a Salpeter stellar inital mass function) and median gas reservoirs of similar to 1.7 x 10(11) M-circle dot. These candidate protoclusters have average total SFRs of at least approximate to(2.3 +/- 0.5) x 10(3) M-circle dot yr(-1) and space densities of similar to 9 x 10(-7) Mpc(-3), consistent with the idea that their constituents may evolve to become massive early-type galaxies in the centers of the rich galaxy clusters we see today.